[PATCHv2 0/2] pahole: Workaround dwarf bug for function encoding

[PATCHv2 0/2] pahole: Workaround dwarf bug for function encoding

[Jiri Olsa]

hi,
because of gcc bug [1] we can no longer rely on DW_AT_declaration
attribute to filter out declarations and end up with just
one copy of the function in the BTF data.

It seems this bug is not easy to fix, but regardless if the
it's coming soon, it's probably good idea not to depend so
much only on dwarf data and make some extra checks.

Thus for function encoding we are now doing following checks:
  - argument names are defined for the function
  - there's symbol and address defined for the function
  - function address belongs to ftrace locations (new in v2)
  - function is generated only once

v2 changes:
  - add check ensuring functions belong to ftrace's mcount
    locations, this way we ensure to have in BTF only
    functions available for ftrace - patch 2 changelog
    describes all details
  - use collect* function names [Andrii]
  - use conventional size increase in realloc [Andrii]
  - drop elf_sym__is_function check
  - drop patch 3, it's not needed, because we follow ftrace
    locations

With v2 patchset I'm getting 38334 BTF functions, which is
reasonable subset of ftrace's available_filter_functions.

thanks,
jirka


[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060
---
Jiri Olsa (2):
      btf_encoder: Move find_all_percpu_vars in generic collect_symbols
      btf_encoder: Change functions check due to broken dwarf

 btf_encoder.c | 338 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 283 insertions(+), 55 deletions(-)

[PATCH 1/2] btf_encoder: Move find_all_percpu_vars in generic collect_symbols

[Jiri Olsa]

Moving find_all_percpu_vars under generic collect_symbols
function that walks over symbols and calls collect_percpu_var.

We will add another collect function that needs to go through
all the symbols, so it's better we go through them just once.

There's no functional change intended.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 btf_encoder.c | 124 +++++++++++++++++++++++++++-----------------------
 1 file changed, 67 insertions(+), 57 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 4c92908beab2..1866bb16a8ba 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -250,75 +250,85 @@ static bool percpu_var_exists(uint64_t addr, uint32_t *sz, const char **name)
 	return true;
 }
 
-static int find_all_percpu_vars(struct btf_elf *btfe)
+static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
 {
-	uint32_t core_id;
-	GElf_Sym sym;
+	const char *sym_name;
+	uint64_t addr;
+	uint32_t size;
 
-	/* cache variables' addresses, preparing for searching in symtab. */
-	percpu_var_cnt = 0;
+	/* compare a symbol's shndx to determine if it's a percpu variable */
+	if (elf_sym__section(sym) != btfe->percpu_shndx)
+		return 0;
+	if (elf_sym__type(sym) != STT_OBJECT)
+		return 0;
 
-	/* search within symtab for percpu variables */
-	elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
-		const char *sym_name;
-		uint64_t addr;
-		uint32_t size;
+	addr = elf_sym__value(sym);
+	/*
+	 * Store only those symbols that have allocated space in the percpu section.
+	 * This excludes the following three types of symbols:
+	 *
+	 *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
+	 *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
+	 *  3. __exitcall(fn), functions which are labeled as exit calls.
+	 *
+	 * In addition, the variables defined using DEFINE_PERCPU_FIRST are
+	 * also not included, which currently includes:
+	 *
+	 *  1. fixed_percpu_data
+	 */
+	if (!addr)
+		return 0;
 
-		/* compare a symbol's shndx to determine if it's a percpu variable */
-		if (elf_sym__section(&sym) != btfe->percpu_shndx)
-			continue;
-		if (elf_sym__type(&sym) != STT_OBJECT)
-			continue;
+	size = elf_sym__size(sym);
+	if (!size)
+		return 0; /* ignore zero-sized symbols */
 
-		addr = elf_sym__value(&sym);
-		/*
-		 * Store only those symbols that have allocated space in the percpu section.
-		 * This excludes the following three types of symbols:
-		 *
-		 *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
-		 *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
-		 *  3. __exitcall(fn), functions which are labeled as exit calls.
-		 *
-		 * In addition, the variables defined using DEFINE_PERCPU_FIRST are
-		 * also not included, which currently includes:
-		 *
-		 *  1. fixed_percpu_data
-		 */
-		if (!addr)
-			continue;
+	sym_name = elf_sym__name(sym, btfe->symtab);
+	if (!btf_name_valid(sym_name)) {
+		dump_invalid_symbol("Found symbol of invalid name when encoding btf",
+				    sym_name, btf_elf__verbose, btf_elf__force);
+		if (btf_elf__force)
+			return 0;
+		return -1;
+	}
 
-		size = elf_sym__size(&sym);
-		if (!size)
-			continue; /* ignore zero-sized symbols */
+	if (btf_elf__verbose)
+		printf("Found per-CPU symbol '%s' at address 0x%lx\n", sym_name, addr);
 
-		sym_name = elf_sym__name(&sym, btfe->symtab);
-		if (!btf_name_valid(sym_name)) {
-			dump_invalid_symbol("Found symbol of invalid name when encoding btf",
-					    sym_name, btf_elf__verbose, btf_elf__force);
-			if (btf_elf__force)
-				continue;
-			return -1;
-		}
+	if (percpu_var_cnt == MAX_PERCPU_VAR_CNT) {
+		fprintf(stderr, "Reached the limit of per-CPU variables: %d\n",
+			MAX_PERCPU_VAR_CNT);
+		return -1;
+	}
+	percpu_vars[percpu_var_cnt].addr = addr;
+	percpu_vars[percpu_var_cnt].sz = size;
+	percpu_vars[percpu_var_cnt].name = sym_name;
+	percpu_var_cnt++;
 
-		if (btf_elf__verbose)
-			printf("Found per-CPU symbol '%s' at address 0x%lx\n", sym_name, addr);
+	return 0;
+}
+
+static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
+{
+	uint32_t core_id;
+	GElf_Sym sym;
 
-		if (percpu_var_cnt == MAX_PERCPU_VAR_CNT) {
-			fprintf(stderr, "Reached the limit of per-CPU variables: %d\n",
-				MAX_PERCPU_VAR_CNT);
+	/* cache variables' addresses, preparing for searching in symtab. */
+	percpu_var_cnt = 0;
+
+	/* search within symtab for percpu variables */
+	elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
+		if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
 			return -1;
-		}
-		percpu_vars[percpu_var_cnt].addr = addr;
-		percpu_vars[percpu_var_cnt].sz = size;
-		percpu_vars[percpu_var_cnt].name = sym_name;
-		percpu_var_cnt++;
 	}
 
-	if (percpu_var_cnt)
-		qsort(percpu_vars, percpu_var_cnt, sizeof(percpu_vars[0]), percpu_var_cmp);
+	if (collect_percpu_vars) {
+		if (percpu_var_cnt)
+			qsort(percpu_vars, percpu_var_cnt, sizeof(percpu_vars[0]), percpu_var_cmp);
 
-	if (btf_elf__verbose)
-		printf("Found %d per-CPU variables!\n", percpu_var_cnt);
+		if (btf_elf__verbose)
+			printf("Found %d per-CPU variables!\n", percpu_var_cnt);
+	}
 	return 0;
 }
 
@@ -347,7 +357,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
 		if (!btfe)
 			return -1;
 
-		if (!skip_encoding_vars && find_all_percpu_vars(btfe))
+		if (collect_symbols(btfe, !skip_encoding_vars))
 			goto out;
 
 		has_index_type = false;
-- 
2.26.2

Re: [PATCH 1/2] btf_encoder: Move find_all_percpu_vars in generic collect_symbols

[Hao Luo]

This looks good to me. Thanks, Jiri.

Acked-by: Hao Luo <haoluo@google.com>

On Sat, Oct 31, 2020 at 3:31 PM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Moving find_all_percpu_vars under generic collect_symbols
> function that walks over symbols and calls collect_percpu_var.
>
> We will add another collect function that needs to go through
> all the symbols, so it's better we go through them just once.
>
> There's no functional change intended.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  btf_encoder.c | 124 +++++++++++++++++++++++++++-----------------------
>  1 file changed, 67 insertions(+), 57 deletions(-)
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 4c92908beab2..1866bb16a8ba 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -250,75 +250,85 @@ static bool percpu_var_exists(uint64_t addr, uint32_t *sz, const char **name)
>         return true;
>  }
>
> -static int find_all_percpu_vars(struct btf_elf *btfe)
> +static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
>  {
> -       uint32_t core_id;
> -       GElf_Sym sym;
> +       const char *sym_name;
> +       uint64_t addr;
> +       uint32_t size;
>
> -       /* cache variables' addresses, preparing for searching in symtab. */
> -       percpu_var_cnt = 0;
> +       /* compare a symbol's shndx to determine if it's a percpu variable */
> +       if (elf_sym__section(sym) != btfe->percpu_shndx)
> +               return 0;
> +       if (elf_sym__type(sym) != STT_OBJECT)
> +               return 0;
>
> -       /* search within symtab for percpu variables */
> -       elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
> -               const char *sym_name;
> -               uint64_t addr;
> -               uint32_t size;
> +       addr = elf_sym__value(sym);
> +       /*
> +        * Store only those symbols that have allocated space in the percpu section.
> +        * This excludes the following three types of symbols:
> +        *
> +        *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
> +        *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
> +        *  3. __exitcall(fn), functions which are labeled as exit calls.
> +        *
> +        * In addition, the variables defined using DEFINE_PERCPU_FIRST are
> +        * also not included, which currently includes:
> +        *
> +        *  1. fixed_percpu_data
> +        */
> +       if (!addr)
> +               return 0;
>
> -               /* compare a symbol's shndx to determine if it's a percpu variable */
> -               if (elf_sym__section(&sym) != btfe->percpu_shndx)
> -                       continue;
> -               if (elf_sym__type(&sym) != STT_OBJECT)
> -                       continue;
> +       size = elf_sym__size(sym);
> +       if (!size)
> +               return 0; /* ignore zero-sized symbols */
>
> -               addr = elf_sym__value(&sym);
> -               /*
> -                * Store only those symbols that have allocated space in the percpu section.
> -                * This excludes the following three types of symbols:
> -                *
> -                *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
> -                *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
> -                *  3. __exitcall(fn), functions which are labeled as exit calls.
> -                *
> -                * In addition, the variables defined using DEFINE_PERCPU_FIRST are
> -                * also not included, which currently includes:
> -                *
> -                *  1. fixed_percpu_data
> -                */
> -               if (!addr)
> -                       continue;
> +       sym_name = elf_sym__name(sym, btfe->symtab);
> +       if (!btf_name_valid(sym_name)) {
> +               dump_invalid_symbol("Found symbol of invalid name when encoding btf",
> +                                   sym_name, btf_elf__verbose, btf_elf__force);
> +               if (btf_elf__force)
> +                       return 0;
> +               return -1;
> +       }
>
> -               size = elf_sym__size(&sym);
> -               if (!size)
> -                       continue; /* ignore zero-sized symbols */
> +       if (btf_elf__verbose)
> +               printf("Found per-CPU symbol '%s' at address 0x%lx\n", sym_name, addr);
>
> -               sym_name = elf_sym__name(&sym, btfe->symtab);
> -               if (!btf_name_valid(sym_name)) {
> -                       dump_invalid_symbol("Found symbol of invalid name when encoding btf",
> -                                           sym_name, btf_elf__verbose, btf_elf__force);
> -                       if (btf_elf__force)
> -                               continue;
> -                       return -1;
> -               }
> +       if (percpu_var_cnt == MAX_PERCPU_VAR_CNT) {
> +               fprintf(stderr, "Reached the limit of per-CPU variables: %d\n",
> +                       MAX_PERCPU_VAR_CNT);
> +               return -1;
> +       }
> +       percpu_vars[percpu_var_cnt].addr = addr;
> +       percpu_vars[percpu_var_cnt].sz = size;
> +       percpu_vars[percpu_var_cnt].name = sym_name;
> +       percpu_var_cnt++;
>
> -               if (btf_elf__verbose)
> -                       printf("Found per-CPU symbol '%s' at address 0x%lx\n", sym_name, addr);
> +       return 0;
> +}
> +
> +static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> +{
> +       uint32_t core_id;
> +       GElf_Sym sym;
>
> -               if (percpu_var_cnt == MAX_PERCPU_VAR_CNT) {
> -                       fprintf(stderr, "Reached the limit of per-CPU variables: %d\n",
> -                               MAX_PERCPU_VAR_CNT);
> +       /* cache variables' addresses, preparing for searching in symtab. */
> +       percpu_var_cnt = 0;
> +
> +       /* search within symtab for percpu variables */
> +       elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
> +               if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
>                         return -1;
> -               }
> -               percpu_vars[percpu_var_cnt].addr = addr;
> -               percpu_vars[percpu_var_cnt].sz = size;
> -               percpu_vars[percpu_var_cnt].name = sym_name;
> -               percpu_var_cnt++;
>         }
>
> -       if (percpu_var_cnt)
> -               qsort(percpu_vars, percpu_var_cnt, sizeof(percpu_vars[0]), percpu_var_cmp);
> +       if (collect_percpu_vars) {
> +               if (percpu_var_cnt)
> +                       qsort(percpu_vars, percpu_var_cnt, sizeof(percpu_vars[0]), percpu_var_cmp);
>
> -       if (btf_elf__verbose)
> -               printf("Found %d per-CPU variables!\n", percpu_var_cnt);
> +               if (btf_elf__verbose)
> +                       printf("Found %d per-CPU variables!\n", percpu_var_cnt);
> +       }
>         return 0;
>  }
>
> @@ -347,7 +357,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>                 if (!btfe)
>                         return -1;
>
> -               if (!skip_encoding_vars && find_all_percpu_vars(btfe))
> +               if (collect_symbols(btfe, !skip_encoding_vars))
>                         goto out;
>
>                 has_index_type = false;
> --
> 2.26.2
>

Re: [PATCH 1/2] btf_encoder: Move find_all_percpu_vars in generic collect_symbols

[Arnaldo Carvalho de Melo]

Em Mon, Nov 02, 2020 at 10:29:22AM -0800, Hao Luo escreveu:
> This looks good to me. Thanks, Jiri.
> 
> Acked-by: Hao Luo <haoluo@google.com>

Thanks, applied, will wait for a v2 for the other.
 
> On Sat, Oct 31, 2020 at 3:31 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Moving find_all_percpu_vars under generic collect_symbols
> > function that walks over symbols and calls collect_percpu_var.
> >
> > We will add another collect function that needs to go through
> > all the symbols, so it's better we go through them just once.
> >
> > There's no functional change intended.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  btf_encoder.c | 124 +++++++++++++++++++++++++++-----------------------
> >  1 file changed, 67 insertions(+), 57 deletions(-)
> >
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index 4c92908beab2..1866bb16a8ba 100644
> > --- a/btf_encoder.c
> > +++ b/btf_encoder.c
> > @@ -250,75 +250,85 @@ static bool percpu_var_exists(uint64_t addr, uint32_t *sz, const char **name)
> >         return true;
> >  }
> >
> > -static int find_all_percpu_vars(struct btf_elf *btfe)
> > +static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
> >  {
> > -       uint32_t core_id;
> > -       GElf_Sym sym;
> > +       const char *sym_name;
> > +       uint64_t addr;
> > +       uint32_t size;
> >
> > -       /* cache variables' addresses, preparing for searching in symtab. */
> > -       percpu_var_cnt = 0;
> > +       /* compare a symbol's shndx to determine if it's a percpu variable */
> > +       if (elf_sym__section(sym) != btfe->percpu_shndx)
> > +               return 0;
> > +       if (elf_sym__type(sym) != STT_OBJECT)
> > +               return 0;
> >
> > -       /* search within symtab for percpu variables */
> > -       elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
> > -               const char *sym_name;
> > -               uint64_t addr;
> > -               uint32_t size;
> > +       addr = elf_sym__value(sym);
> > +       /*
> > +        * Store only those symbols that have allocated space in the percpu section.
> > +        * This excludes the following three types of symbols:
> > +        *
> > +        *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
> > +        *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
> > +        *  3. __exitcall(fn), functions which are labeled as exit calls.
> > +        *
> > +        * In addition, the variables defined using DEFINE_PERCPU_FIRST are
> > +        * also not included, which currently includes:
> > +        *
> > +        *  1. fixed_percpu_data
> > +        */
> > +       if (!addr)
> > +               return 0;
> >
> > -               /* compare a symbol's shndx to determine if it's a percpu variable */
> > -               if (elf_sym__section(&sym) != btfe->percpu_shndx)
> > -                       continue;
> > -               if (elf_sym__type(&sym) != STT_OBJECT)
> > -                       continue;
> > +       size = elf_sym__size(sym);
> > +       if (!size)
> > +               return 0; /* ignore zero-sized symbols */
> >
> > -               addr = elf_sym__value(&sym);
> > -               /*
> > -                * Store only those symbols that have allocated space in the percpu section.
> > -                * This excludes the following three types of symbols:
> > -                *
> > -                *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
> > -                *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
> > -                *  3. __exitcall(fn), functions which are labeled as exit calls.
> > -                *
> > -                * In addition, the variables defined using DEFINE_PERCPU_FIRST are
> > -                * also not included, which currently includes:
> > -                *
> > -                *  1. fixed_percpu_data
> > -                */
> > -               if (!addr)
> > -                       continue;
> > +       sym_name = elf_sym__name(sym, btfe->symtab);
> > +       if (!btf_name_valid(sym_name)) {
> > +               dump_invalid_symbol("Found symbol of invalid name when encoding btf",
> > +                                   sym_name, btf_elf__verbose, btf_elf__force);
> > +               if (btf_elf__force)
> > +                       return 0;
> > +               return -1;
> > +       }
> >
> > -               size = elf_sym__size(&sym);
> > -               if (!size)
> > -                       continue; /* ignore zero-sized symbols */
> > +       if (btf_elf__verbose)
> > +               printf("Found per-CPU symbol '%s' at address 0x%lx\n", sym_name, addr);
> >
> > -               sym_name = elf_sym__name(&sym, btfe->symtab);
> > -               if (!btf_name_valid(sym_name)) {
> > -                       dump_invalid_symbol("Found symbol of invalid name when encoding btf",
> > -                                           sym_name, btf_elf__verbose, btf_elf__force);
> > -                       if (btf_elf__force)
> > -                               continue;
> > -                       return -1;
> > -               }
> > +       if (percpu_var_cnt == MAX_PERCPU_VAR_CNT) {
> > +               fprintf(stderr, "Reached the limit of per-CPU variables: %d\n",
> > +                       MAX_PERCPU_VAR_CNT);
> > +               return -1;
> > +       }
> > +       percpu_vars[percpu_var_cnt].addr = addr;
> > +       percpu_vars[percpu_var_cnt].sz = size;
> > +       percpu_vars[percpu_var_cnt].name = sym_name;
> > +       percpu_var_cnt++;
> >
> > -               if (btf_elf__verbose)
> > -                       printf("Found per-CPU symbol '%s' at address 0x%lx\n", sym_name, addr);
> > +       return 0;
> > +}
> > +
> > +static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> > +{
> > +       uint32_t core_id;
> > +       GElf_Sym sym;
> >
> > -               if (percpu_var_cnt == MAX_PERCPU_VAR_CNT) {
> > -                       fprintf(stderr, "Reached the limit of per-CPU variables: %d\n",
> > -                               MAX_PERCPU_VAR_CNT);
> > +       /* cache variables' addresses, preparing for searching in symtab. */
> > +       percpu_var_cnt = 0;
> > +
> > +       /* search within symtab for percpu variables */
> > +       elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
> > +               if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
> >                         return -1;
> > -               }
> > -               percpu_vars[percpu_var_cnt].addr = addr;
> > -               percpu_vars[percpu_var_cnt].sz = size;
> > -               percpu_vars[percpu_var_cnt].name = sym_name;
> > -               percpu_var_cnt++;
> >         }
> >
> > -       if (percpu_var_cnt)
> > -               qsort(percpu_vars, percpu_var_cnt, sizeof(percpu_vars[0]), percpu_var_cmp);
> > +       if (collect_percpu_vars) {
> > +               if (percpu_var_cnt)
> > +                       qsort(percpu_vars, percpu_var_cnt, sizeof(percpu_vars[0]), percpu_var_cmp);
> >
> > -       if (btf_elf__verbose)
> > -               printf("Found %d per-CPU variables!\n", percpu_var_cnt);
> > +               if (btf_elf__verbose)
> > +                       printf("Found %d per-CPU variables!\n", percpu_var_cnt);
> > +       }
> >         return 0;
> >  }
> >
> > @@ -347,7 +357,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> >                 if (!btfe)
> >                         return -1;
> >
> > -               if (!skip_encoding_vars && find_all_percpu_vars(btfe))
> > +               if (collect_symbols(btfe, !skip_encoding_vars))
> >                         goto out;
> >
> >                 has_index_type = false;
> > --
> > 2.26.2
> >

-- 

- Arnaldo

[PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa]

We need to generate just single BTF instance for the
function, while DWARF data contains multiple instances
of DW_TAG_subprogram tag.

Unfortunately we can no longer rely on DW_AT_declaration
tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)

Instead we apply following checks:
  - argument names are defined for the function
  - there's symbol and address defined for the function
  - function is generated only once

Also because we want to follow kernel's ftrace traceable
functions, this patchset is adding extra check that the
function is one of the ftrace's functions.

All ftrace functions addresses are stored in vmlinux
binary within symbols:
  __start_mcount_loc
  __stop_mcount_loc

During object preparation code we read those addresses,
sort them and use them as filter for all detected dwarf
functions.

We also filter out functions within .init section, ftrace
is doing that in runtime.

I can still see several differences to ftrace functions in
/sys/kernel/debug/tracing/available_filter_functions file:

  - available_filter_functions includes modules (7086 functions)
  - available_filter_functions includes functions like:
      __acpi_match_device.part.0.constprop.0
      acpi_ns_check_sorted_list.constprop.0
      acpi_os_unmap_generic_address.part.0
      acpiphp_check_bridge.part.0

    which are not part of dwarf data (1164 functions)
  - BTF includes multiple functions like:
      __clk_register_clkdev
      clk_register_clkdev

    which share same code so they appear just as single function
    in available_filter_functions, but dwarf keeps track of both
    of them (16 functions)

With this change I'm getting 38334 BTF functions, which
when added above functions to consideration gives same
amount of functions in available_filter_functions.

The patch still keeps the original function filter condition
(that uses current fn->declaration check) in case the object
does not contain *_mcount_loc symbol -> object is not vmlinux.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 btf_encoder.c | 222 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 220 insertions(+), 2 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 1866bb16a8ba..0a378aa92142 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -26,6 +26,151 @@
  */
 #define KSYM_NAME_LEN 128
 
+struct mcount_symbols {
+	unsigned long start;
+	unsigned long stop;
+	unsigned long init_begin;
+	unsigned long init_end;
+	unsigned long start_section;
+};
+
+struct elf_function {
+	const char	*name;
+	unsigned long	 addr;
+	bool		 generated;
+	bool		 valid;
+};
+
+static struct elf_function *functions;
+static int functions_alloc;
+static int functions_cnt;
+static int functions_valid;
+
+static int functions_cmp(const void *_a, const void *_b)
+{
+	const struct elf_function *a = _a;
+	const struct elf_function *b = _b;
+
+	return strcmp(a->name, b->name);
+}
+
+static void delete_functions(void)
+{
+	free(functions);
+	functions_alloc = functions_cnt = functions_valid = 0;
+}
+
+#ifndef max
+# define max(x, y) ((x) < (y) ? (y) : (x))
+#endif
+
+static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
+{
+	if (elf_sym__type(sym) != STT_FUNC)
+		return 0;
+	if (!elf_sym__value(sym))
+		return 0;
+
+	if (functions_cnt == functions_alloc) {
+		functions_alloc = max(1000, functions_alloc * 3 / 2);
+		functions = realloc(functions, functions_alloc * sizeof(*functions));
+		if (!functions)
+			return -1;
+	}
+
+	functions[functions_cnt].name = elf_sym__name(sym, btfe->symtab);
+	functions[functions_cnt].addr = elf_sym__value(sym);
+	functions[functions_cnt].generated = false;
+	functions[functions_cnt].valid = false;
+	functions_cnt++;
+	return 0;
+}
+
+static int addrs_cmp(const void *_a, const void *_b)
+{
+	const unsigned long *a = _a;
+	const unsigned long *b = _b;
+
+	return *a - *b;
+}
+
+static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
+{
+	bool init_filter = ms->init_begin && ms->init_end;
+	unsigned long *addrs, count, offset, i;
+	Elf_Data *data;
+	GElf_Shdr shdr;
+	Elf_Scn *sec;
+
+	/*
+	 * Find mcount addressed marked by __start_mcount_loc
+	 * and __stop_mcount_loc symbols and load them into
+	 * sorted array.
+	 */
+	sec = elf_getscn(btfe->elf, ms->start_section);
+	if (!sec || !gelf_getshdr(sec, &shdr)) {
+		fprintf(stderr, "Failed to get section(%lu) header.\n",
+			ms->start_section);
+		return -1;
+	}
+
+	offset = ms->start - shdr.sh_addr;
+	count  = (ms->stop - ms->start) / 8;
+
+	data = elf_getdata(sec, 0);
+	if (!data) {
+		fprintf(stderr, "Failed to section(%lu) data.\n",
+			ms->start_section);
+		return -1;
+	}
+
+	addrs = malloc(count * sizeof(addrs[0]));
+	if (!addrs) {
+		fprintf(stderr, "Failed to allocate memory for ftrace addresses.\n");
+		return -1;
+	}
+
+	memcpy(addrs, data->d_buf + offset, count * sizeof(addrs[0]));
+	qsort(addrs, count, sizeof(addrs[0]), addrs_cmp);
+
+	/*
+	 * Let's got through all collected functions and filter
+	 * out those that are not in ftrace and init code.
+	 */
+	for (i = 0; i < functions_cnt; i++) {
+		struct elf_function *func = &functions[i];
+
+		/* Do not enable .init section functions. */
+		if (init_filter &&
+		    func->addr >= ms->init_begin &&
+		    func->addr <  ms->init_end)
+			continue;
+
+		/* Make sure function is within mcount addresses. */
+		if (bsearch(&func->addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
+			functions_valid++;
+			func->valid = true;
+		}
+	}
+
+	free(addrs);
+	return 0;
+}
+
+static bool should_generate_function(const struct btf_elf *btfe, const char *name)
+{
+	struct elf_function *p;
+	struct elf_function key = { .name = name };
+
+	p = bsearch(&key, functions, functions_cnt,
+		    sizeof(functions[0]), functions_cmp);
+	if (!p || !p->valid || p->generated)
+		return false;
+
+	p->generated = true;
+	return true;
+}
+
 static bool btf_name_char_ok(char c, bool first)
 {
 	if (c == '_' || c == '.')
@@ -207,6 +352,7 @@ int btf_encoder__encode()
 		btf_elf__add_datasec_type(btfe, PERCPU_SECTION, &btfe->percpu_secinfo);
 
 	err = btf_elf__encode(btfe, 0);
+	delete_functions();
 	btf_elf__delete(btfe);
 	btfe = NULL;
 
@@ -308,8 +454,27 @@ static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
 	return 0;
 }
 
+#define SET_SYMBOL(__sym, __var)						\
+	if (!ms->__var && !strcmp(__sym, elf_sym__name(sym, btfe->symtab)))	\
+		ms->__var = sym->st_value;					\
+
+static void collect_mcount_symbol(GElf_Sym *sym, struct mcount_symbols *ms)
+{
+	if (!ms->start &&
+	    !strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) {
+		ms->start = sym->st_value;
+		ms->start_section = sym->st_shndx;
+	}
+	SET_SYMBOL("__stop_mcount_loc", stop)
+	SET_SYMBOL("__init_begin", init_begin)
+	SET_SYMBOL("__init_end", init_end)
+}
+
+#undef SET_SYMBOL
+
 static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
 {
+	struct mcount_symbols ms = { };
 	uint32_t core_id;
 	GElf_Sym sym;
 
@@ -320,6 +485,9 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
 	elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
 		if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
 			return -1;
+		if (collect_function(btfe, &sym))
+			return -1;
+		collect_mcount_symbol(&sym, &ms);
 	}
 
 	if (collect_percpu_vars) {
@@ -329,9 +497,34 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
 		if (btf_elf__verbose)
 			printf("Found %d per-CPU variables!\n", percpu_var_cnt);
 	}
+
+	if (functions_cnt) {
+		qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp);
+		if (ms.start && ms.stop &&
+		    filter_functions(btfe, &ms)) {
+			fprintf(stderr, "Failed to filter dwarf functions\n");
+			return -1;
+		}
+		if (btf_elf__verbose)
+			printf("Found %d functions!\n", functions_valid);
+	}
+
 	return 0;
 }
 
+static bool has_arg_names(struct cu *cu, struct ftype *ftype)
+{
+	struct parameter *param;
+	const char *name;
+
+	ftype__for_each_parameter(ftype, param) {
+		name = dwarves__active_loader->strings__ptr(cu, param->name);
+		if (name == NULL)
+			return false;
+	}
+	return true;
+}
+
 int cu__encode_btf(struct cu *cu, int verbose, bool force,
 		   bool skip_encoding_vars)
 {
@@ -407,8 +600,32 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
 		int btf_fnproto_id, btf_fn_id;
 		const char *name;
 
-		if (fn->declaration || !fn->external)
-			continue;
+		/*
+		 * The functions_valid != 0 means we parsed all necessary
+		 * kernel symbols and we are using mcount location filter
+		 * for functions. If it's not available keep the current
+		 * dwarf declaration check.
+		 */
+		if (functions_valid) {
+			/*
+			 * We need to generate just single BTF instance for the
+			 * function, while DWARF data contains multiple instances
+			 * of DW_TAG_subprogram tag.
+			 *
+			 * We check following conditions:
+			 *   - argument names are defined
+			 *   - there's symbol and address defined for the function
+			 *   - function address belongs to ftrace locations
+			 *   - function is generated only once
+			 */
+			if (!has_arg_names(cu, &fn->proto))
+				continue;
+			if (!should_generate_function(btfe, function__name(fn, cu)))
+				continue;
+		} else {
+			if (fn->declaration || !fn->external)
+				continue;
+		}
 
 		btf_fnproto_id = btf_elf__add_func_proto(btfe, cu, &fn->proto, type_id_off);
 		name = dwarves__active_loader->strings__ptr(cu, fn->name);
@@ -492,6 +709,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
 
 out:
 	if (err) {
+		delete_functions();
 		btf_elf__delete(btfe);
 		btfe = NULL;
 	}
-- 
2.26.2

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa]

On Sat, Oct 31, 2020 at 11:31:31PM +0100, Jiri Olsa wrote:
> We need to generate just single BTF instance for the
> function, while DWARF data contains multiple instances
> of DW_TAG_subprogram tag.
> 
> Unfortunately we can no longer rely on DW_AT_declaration
> tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
> 
> Instead we apply following checks:
>   - argument names are defined for the function
>   - there's symbol and address defined for the function
>   - function is generated only once
> 
> Also because we want to follow kernel's ftrace traceable
> functions, this patchset is adding extra check that the
> function is one of the ftrace's functions.
> 
> All ftrace functions addresses are stored in vmlinux
> binary within symbols:
>   __start_mcount_loc
>   __stop_mcount_loc

hum, for some reason this does not pass through bpf internal
functions like bpf_iter_bpf_map.. I learned it hard way ;-)
will check

jirka

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa]

On Mon, Nov 02, 2020 at 10:59:08PM +0100, Jiri Olsa wrote:
> On Sat, Oct 31, 2020 at 11:31:31PM +0100, Jiri Olsa wrote:
> > We need to generate just single BTF instance for the
> > function, while DWARF data contains multiple instances
> > of DW_TAG_subprogram tag.
> > 
> > Unfortunately we can no longer rely on DW_AT_declaration
> > tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
> > 
> > Instead we apply following checks:
> >   - argument names are defined for the function
> >   - there's symbol and address defined for the function
> >   - function is generated only once
> > 
> > Also because we want to follow kernel's ftrace traceable
> > functions, this patchset is adding extra check that the
> > function is one of the ftrace's functions.
> > 
> > All ftrace functions addresses are stored in vmlinux
> > binary within symbols:
> >   __start_mcount_loc
> >   __stop_mcount_loc
> 
> hum, for some reason this does not pass through bpf internal
> functions like bpf_iter_bpf_map.. I learned it hard way ;-)
> will check

so it gets filtered out because it's __init function
I'll check if the fix below catches all internal functions,
but I guess we should do something more robust

jirka


---
diff --git a/btf_encoder.c b/btf_encoder.c
index 0a378aa92142..3cd94280c35b 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -143,7 +143,8 @@ static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
 		/* Do not enable .init section functions. */
 		if (init_filter &&
 		    func->addr >= ms->init_begin &&
-		    func->addr <  ms->init_end)
+		    func->addr <  ms->init_end &&
+		    strncmp("bpf_", func->name, 4))
 			continue;
 
 		/* Make sure function is within mcount addresses. */

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Andrii Nakryiko]

On Mon, Nov 2, 2020 at 2:57 PM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Mon, Nov 02, 2020 at 10:59:08PM +0100, Jiri Olsa wrote:
> > On Sat, Oct 31, 2020 at 11:31:31PM +0100, Jiri Olsa wrote:
> > > We need to generate just single BTF instance for the
> > > function, while DWARF data contains multiple instances
> > > of DW_TAG_subprogram tag.
> > >
> > > Unfortunately we can no longer rely on DW_AT_declaration
> > > tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
> > >
> > > Instead we apply following checks:
> > >   - argument names are defined for the function
> > >   - there's symbol and address defined for the function
> > >   - function is generated only once
> > >
> > > Also because we want to follow kernel's ftrace traceable
> > > functions, this patchset is adding extra check that the
> > > function is one of the ftrace's functions.
> > >
> > > All ftrace functions addresses are stored in vmlinux
> > > binary within symbols:
> > >   __start_mcount_loc
> > >   __stop_mcount_loc
> >
> > hum, for some reason this does not pass through bpf internal
> > functions like bpf_iter_bpf_map.. I learned it hard way ;-)

what's the exact name of the function that was missing?
bpf_iter_bpf_map doesn't exist. And if it's __init function, why does
it matter, it's not going to be even available at runtime, right?


> > will check
>
> so it gets filtered out because it's __init function
> I'll check if the fix below catches all internal functions,
> but I guess we should do something more robust
>
> jirka
>
>
> ---
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 0a378aa92142..3cd94280c35b 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -143,7 +143,8 @@ static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
>                 /* Do not enable .init section functions. */
>                 if (init_filter &&
>                     func->addr >= ms->init_begin &&
> -                   func->addr <  ms->init_end)
> +                   func->addr <  ms->init_end &&
> +                   strncmp("bpf_", func->name, 4))

this looks like a very wrong way to do this? Can you please elaborate
on what's missing and why it shouldn't be missing?

>                         continue;
>
>                 /* Make sure function is within mcount addresses. */
>

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa]

On Tue, Nov 03, 2020 at 10:58:58AM -0800, Andrii Nakryiko wrote:
> On Mon, Nov 2, 2020 at 2:57 PM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Mon, Nov 02, 2020 at 10:59:08PM +0100, Jiri Olsa wrote:
> > > On Sat, Oct 31, 2020 at 11:31:31PM +0100, Jiri Olsa wrote:
> > > > We need to generate just single BTF instance for the
> > > > function, while DWARF data contains multiple instances
> > > > of DW_TAG_subprogram tag.
> > > >
> > > > Unfortunately we can no longer rely on DW_AT_declaration
> > > > tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
> > > >
> > > > Instead we apply following checks:
> > > >   - argument names are defined for the function
> > > >   - there's symbol and address defined for the function
> > > >   - function is generated only once
> > > >
> > > > Also because we want to follow kernel's ftrace traceable
> > > > functions, this patchset is adding extra check that the
> > > > function is one of the ftrace's functions.
> > > >
> > > > All ftrace functions addresses are stored in vmlinux
> > > > binary within symbols:
> > > >   __start_mcount_loc
> > > >   __stop_mcount_loc
> > >
> > > hum, for some reason this does not pass through bpf internal
> > > functions like bpf_iter_bpf_map.. I learned it hard way ;-)
> 
> what's the exact name of the function that was missing?
> bpf_iter_bpf_map doesn't exist. And if it's __init function, why does
> it matter, it's not going to be even available at runtime, right?
> 

bpf_map iter definition:

DEFINE_BPF_ITER_FUNC(bpf_map, struct bpf_iter_meta *meta, struct bpf_map *map)

goes to:

#define DEFINE_BPF_ITER_FUNC(target, args...)                   \
        extern int bpf_iter_ ## target(args);                   \
        int __init bpf_iter_ ## target(args) { return 0; }

that creates __init bpf_iter_bpf_map function that will make
it into BTF where it's expected when opening iterator, but the
code will be freed because it's __init function

there are few iteratos functions like that, and I was going to
check if there's more

> 
> > > will check
> >
> > so it gets filtered out because it's __init function
> > I'll check if the fix below catches all internal functions,
> > but I guess we should do something more robust
> >
> > jirka
> >
> >
> > ---
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index 0a378aa92142..3cd94280c35b 100644
> > --- a/btf_encoder.c
> > +++ b/btf_encoder.c
> > @@ -143,7 +143,8 @@ static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
> >                 /* Do not enable .init section functions. */
> >                 if (init_filter &&
> >                     func->addr >= ms->init_begin &&
> > -                   func->addr <  ms->init_end)
> > +                   func->addr <  ms->init_end &&
> > +                   strncmp("bpf_", func->name, 4))
> 
> this looks like a very wrong way to do this? Can you please elaborate
> on what's missing and why it shouldn't be missing?

yes, it's just a hack, we should do something more
robust as I mentioned above

it just allowed me to use iterators finaly ;-)

jirka

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Andrii Nakryiko]

On Tue, Nov 3, 2020 at 11:06 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Tue, Nov 03, 2020 at 10:58:58AM -0800, Andrii Nakryiko wrote:
> > On Mon, Nov 2, 2020 at 2:57 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > >
> > > On Mon, Nov 02, 2020 at 10:59:08PM +0100, Jiri Olsa wrote:
> > > > On Sat, Oct 31, 2020 at 11:31:31PM +0100, Jiri Olsa wrote:
> > > > > We need to generate just single BTF instance for the
> > > > > function, while DWARF data contains multiple instances
> > > > > of DW_TAG_subprogram tag.
> > > > >
> > > > > Unfortunately we can no longer rely on DW_AT_declaration
> > > > > tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
> > > > >
> > > > > Instead we apply following checks:
> > > > >   - argument names are defined for the function
> > > > >   - there's symbol and address defined for the function
> > > > >   - function is generated only once
> > > > >
> > > > > Also because we want to follow kernel's ftrace traceable
> > > > > functions, this patchset is adding extra check that the
> > > > > function is one of the ftrace's functions.
> > > > >
> > > > > All ftrace functions addresses are stored in vmlinux
> > > > > binary within symbols:
> > > > >   __start_mcount_loc
> > > > >   __stop_mcount_loc
> > > >
> > > > hum, for some reason this does not pass through bpf internal
> > > > functions like bpf_iter_bpf_map.. I learned it hard way ;-)
> >
> > what's the exact name of the function that was missing?
> > bpf_iter_bpf_map doesn't exist. And if it's __init function, why does
> > it matter, it's not going to be even available at runtime, right?
> >
>
> bpf_map iter definition:
>
> DEFINE_BPF_ITER_FUNC(bpf_map, struct bpf_iter_meta *meta, struct bpf_map *map)
>
> goes to:
>
> #define DEFINE_BPF_ITER_FUNC(target, args...)                   \
>         extern int bpf_iter_ ## target(args);                   \
>         int __init bpf_iter_ ## target(args) { return 0; }
>
> that creates __init bpf_iter_bpf_map function that will make
> it into BTF where it's expected when opening iterator, but the
> code will be freed because it's __init function

hm... should we just drop __init there?

Yonghong, is __init strictly necessary, or was just an optimization to
save a tiny bit of space?

>
> there are few iteratos functions like that, and I was going to
> check if there's more
>
> >
> > > > will check
> > >
> > > so it gets filtered out because it's __init function
> > > I'll check if the fix below catches all internal functions,
> > > but I guess we should do something more robust
> > >
> > > jirka
> > >
> > >
> > > ---
> > > diff --git a/btf_encoder.c b/btf_encoder.c
> > > index 0a378aa92142..3cd94280c35b 100644
> > > --- a/btf_encoder.c
> > > +++ b/btf_encoder.c
> > > @@ -143,7 +143,8 @@ static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
> > >                 /* Do not enable .init section functions. */
> > >                 if (init_filter &&
> > >                     func->addr >= ms->init_begin &&
> > > -                   func->addr <  ms->init_end)
> > > +                   func->addr <  ms->init_end &&
> > > +                   strncmp("bpf_", func->name, 4))
> >
> > this looks like a very wrong way to do this? Can you please elaborate
> > on what's missing and why it shouldn't be missing?
>
> yes, it's just a hack, we should do something more
> robust as I mentioned above
>
> it just allowed me to use iterators finaly ;-)

sure, I get it, I was just trying to understand why there is such a
problem in the first place. Turns out we need FUNCs not just for
fentry/fexit and similar, but also for bpf_iter, which is an entirely
different use case (similar to raw_tp, but raw_tp is using typedef ->
func_proto approach).

So I don't know, we might as well just not do mcount checks?.. As an
alternative, but it's not great as well.

>
> jirka
>

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa]

On Tue, Nov 03, 2020 at 11:23:33AM -0800, Andrii Nakryiko wrote:

SNIP

> > > on what's missing and why it shouldn't be missing?
> >
> > yes, it's just a hack, we should do something more
> > robust as I mentioned above
> >
> > it just allowed me to use iterators finaly ;-)
> 
> sure, I get it, I was just trying to understand why there is such a
> problem in the first place. Turns out we need FUNCs not just for
> fentry/fexit and similar, but also for bpf_iter, which is an entirely
> different use case (similar to raw_tp, but raw_tp is using typedef ->
> func_proto approach).
> 
> So I don't know, we might as well just not do mcount checks?.. As an
> alternative, but it's not great as well.

how about moving all such functions to separate new .init.XXX section,
and pahole would make one extr acheck for that.. and it still gets freed

jirka

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Yonghong Song]

On 11/3/20 11:23 AM, Andrii Nakryiko wrote:
> On Tue, Nov 3, 2020 at 11:06 AM Jiri Olsa <jolsa@redhat.com> wrote:
>>
>> On Tue, Nov 03, 2020 at 10:58:58AM -0800, Andrii Nakryiko wrote:
>>> On Mon, Nov 2, 2020 at 2:57 PM Jiri Olsa <jolsa@redhat.com> wrote:
>>>>
>>>> On Mon, Nov 02, 2020 at 10:59:08PM +0100, Jiri Olsa wrote:
>>>>> On Sat, Oct 31, 2020 at 11:31:31PM +0100, Jiri Olsa wrote:
>>>>>> We need to generate just single BTF instance for the
>>>>>> function, while DWARF data contains multiple instances
>>>>>> of DW_TAG_subprogram tag.
>>>>>>
>>>>>> Unfortunately we can no longer rely on DW_AT_declaration
>>>>>> tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
>>>>>>
>>>>>> Instead we apply following checks:
>>>>>>    - argument names are defined for the function
>>>>>>    - there's symbol and address defined for the function
>>>>>>    - function is generated only once
>>>>>>
>>>>>> Also because we want to follow kernel's ftrace traceable
>>>>>> functions, this patchset is adding extra check that the
>>>>>> function is one of the ftrace's functions.
>>>>>>
>>>>>> All ftrace functions addresses are stored in vmlinux
>>>>>> binary within symbols:
>>>>>>    __start_mcount_loc
>>>>>>    __stop_mcount_loc
>>>>>
>>>>> hum, for some reason this does not pass through bpf internal
>>>>> functions like bpf_iter_bpf_map.. I learned it hard way ;-)
>>>
>>> what's the exact name of the function that was missing?
>>> bpf_iter_bpf_map doesn't exist. And if it's __init function, why does
>>> it matter, it's not going to be even available at runtime, right?
>>>
>>
>> bpf_map iter definition:
>>
>> DEFINE_BPF_ITER_FUNC(bpf_map, struct bpf_iter_meta *meta, struct bpf_map *map)
>>
>> goes to:
>>
>> #define DEFINE_BPF_ITER_FUNC(target, args...)                   \
>>          extern int bpf_iter_ ## target(args);                   \
>>          int __init bpf_iter_ ## target(args) { return 0; }
>>
>> that creates __init bpf_iter_bpf_map function that will make
>> it into BTF where it's expected when opening iterator, but the
>> code will be freed because it's __init function
> 
> hm... should we just drop __init there?
> 
> Yonghong, is __init strictly necessary, or was just an optimization to
> save a tiny bit of space?

It is an optimization to save some space. We only need function
signature, not function body, for bpf_iter.

The macro definition is in include/linux/bpf.h.

#define DEFINE_BPF_ITER_FUNC(target, args...)                   \
         extern int bpf_iter_ ## target(args);                   \
         int __init bpf_iter_ ## target(args) { return 0; }

Maybe you could have a section, e.g., called
   .init.bpf.preserve_type
which you can scan through to preserve the types.

Alternatively you can drop the above __init, the saving is
indeed tiny. But this adds overhead to ksymbol lookup and
may not be desirable.

> 
>>
>> there are few iteratos functions like that, and I was going to
>> check if there's more
>>
>>>
>>>>> will check
>>>>
>>>> so it gets filtered out because it's __init function
>>>> I'll check if the fix below catches all internal functions,
>>>> but I guess we should do something more robust
>>>>
>>>> jirka
>>>>
>>>>
>>>> ---
>>>> diff --git a/btf_encoder.c b/btf_encoder.c
>>>> index 0a378aa92142..3cd94280c35b 100644
>>>> --- a/btf_encoder.c
>>>> +++ b/btf_encoder.c
>>>> @@ -143,7 +143,8 @@ static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
>>>>                  /* Do not enable .init section functions. */
>>>>                  if (init_filter &&
>>>>                      func->addr >= ms->init_begin &&
>>>> -                   func->addr <  ms->init_end)
>>>> +                   func->addr <  ms->init_end &&
>>>> +                   strncmp("bpf_", func->name, 4))
>>>
>>> this looks like a very wrong way to do this? Can you please elaborate
>>> on what's missing and why it shouldn't be missing?
>>
>> yes, it's just a hack, we should do something more
>> robust as I mentioned above
>>
>> it just allowed me to use iterators finaly ;-)
> 
> sure, I get it, I was just trying to understand why there is such a
> problem in the first place. Turns out we need FUNCs not just for
> fentry/fexit and similar, but also for bpf_iter, which is an entirely
> different use case (similar to raw_tp, but raw_tp is using typedef ->
> func_proto approach).
> 
> So I don't know, we might as well just not do mcount checks?.. As an
> alternative, but it's not great as well.
> 
>>
>> jirka
>>

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa]

On Tue, Nov 03, 2020 at 12:27:56PM -0800, Yonghong Song wrote:

SNIP

> > > > 
> > > 
> > > bpf_map iter definition:
> > > 
> > > DEFINE_BPF_ITER_FUNC(bpf_map, struct bpf_iter_meta *meta, struct bpf_map *map)
> > > 
> > > goes to:
> > > 
> > > #define DEFINE_BPF_ITER_FUNC(target, args...)                   \
> > >          extern int bpf_iter_ ## target(args);                   \
> > >          int __init bpf_iter_ ## target(args) { return 0; }
> > > 
> > > that creates __init bpf_iter_bpf_map function that will make
> > > it into BTF where it's expected when opening iterator, but the
> > > code will be freed because it's __init function
> > 
> > hm... should we just drop __init there?
> > 
> > Yonghong, is __init strictly necessary, or was just an optimization to
> > save a tiny bit of space?
> 
> It is an optimization to save some space. We only need function
> signature, not function body, for bpf_iter.
> 
> The macro definition is in include/linux/bpf.h.
> 
> #define DEFINE_BPF_ITER_FUNC(target, args...)                   \
>         extern int bpf_iter_ ## target(args);                   \
>         int __init bpf_iter_ ## target(args) { return 0; }
> 
> Maybe you could have a section, e.g., called
>   .init.bpf.preserve_type
> which you can scan through to preserve the types.

right, sounds good, will send v3 with that

thanks,
jirka

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Andrii Nakryiko]

On Sat, Oct 31, 2020 at 3:32 PM Jiri Olsa <jolsa@kernel.org> wrote:
>
> We need to generate just single BTF instance for the
> function, while DWARF data contains multiple instances
> of DW_TAG_subprogram tag.
>
> Unfortunately we can no longer rely on DW_AT_declaration
> tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
>
> Instead we apply following checks:
>   - argument names are defined for the function
>   - there's symbol and address defined for the function
>   - function is generated only once
>
> Also because we want to follow kernel's ftrace traceable
> functions, this patchset is adding extra check that the
> function is one of the ftrace's functions.
>
> All ftrace functions addresses are stored in vmlinux
> binary within symbols:
>   __start_mcount_loc
>   __stop_mcount_loc
>
> During object preparation code we read those addresses,
> sort them and use them as filter for all detected dwarf
> functions.
>
> We also filter out functions within .init section, ftrace
> is doing that in runtime.
>
> I can still see several differences to ftrace functions in
> /sys/kernel/debug/tracing/available_filter_functions file:
>
>   - available_filter_functions includes modules (7086 functions)
>   - available_filter_functions includes functions like:
>       __acpi_match_device.part.0.constprop.0
>       acpi_ns_check_sorted_list.constprop.0
>       acpi_os_unmap_generic_address.part.0
>       acpiphp_check_bridge.part.0
>
>     which are not part of dwarf data (1164 functions)
>   - BTF includes multiple functions like:
>       __clk_register_clkdev
>       clk_register_clkdev
>
>     which share same code so they appear just as single function
>     in available_filter_functions, but dwarf keeps track of both
>     of them (16 functions)
>
> With this change I'm getting 38334 BTF functions, which
> when added above functions to consideration gives same
> amount of functions in available_filter_functions.
>
> The patch still keeps the original function filter condition
> (that uses current fn->declaration check) in case the object
> does not contain *_mcount_loc symbol -> object is not vmlinux.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  btf_encoder.c | 222 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 220 insertions(+), 2 deletions(-)

[...]

> +static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
> +{
> +       if (elf_sym__type(sym) != STT_FUNC)
> +               return 0;
> +       if (!elf_sym__value(sym))
> +               return 0;
> +
> +       if (functions_cnt == functions_alloc) {
> +               functions_alloc = max(1000, functions_alloc * 3 / 2);
> +               functions = realloc(functions, functions_alloc * sizeof(*functions));
> +               if (!functions)
> +                       return -1;

memory leak right here. You need to use a temporary variable and check
if for NULL, before overwriting functions.

> +       }
> +
> +       functions[functions_cnt].name = elf_sym__name(sym, btfe->symtab);
> +       functions[functions_cnt].addr = elf_sym__value(sym);
> +       functions[functions_cnt].generated = false;
> +       functions[functions_cnt].valid = false;
> +       functions_cnt++;
> +       return 0;
> +}
> +
> +static int addrs_cmp(const void *_a, const void *_b)
> +{
> +       const unsigned long *a = _a;
> +       const unsigned long *b = _b;
> +
> +       return *a - *b;

this is cute, but is it always correct? instead of thinking how this
works with overflows, maybe let's keep it simple with

if (*a == *b)
  return 0;
return *a < *b ? -1 : 1;

?

> +}
> +
> +static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
> +{
> +       bool init_filter = ms->init_begin && ms->init_end;
> +       unsigned long *addrs, count, offset, i;
> +       Elf_Data *data;
> +       GElf_Shdr shdr;
> +       Elf_Scn *sec;
> +
> +       /*
> +        * Find mcount addressed marked by __start_mcount_loc
> +        * and __stop_mcount_loc symbols and load them into
> +        * sorted array.
> +        */
> +       sec = elf_getscn(btfe->elf, ms->start_section);
> +       if (!sec || !gelf_getshdr(sec, &shdr)) {
> +               fprintf(stderr, "Failed to get section(%lu) header.\n",
> +                       ms->start_section);
> +               return -1;
> +       }
> +
> +       offset = ms->start - shdr.sh_addr;
> +       count  = (ms->stop - ms->start) / 8;
> +
> +       data = elf_getdata(sec, 0);
> +       if (!data) {
> +               fprintf(stderr, "Failed to section(%lu) data.\n",

typo: failed to get?

> +                       ms->start_section);
> +               return -1;
> +       }
> +
> +       addrs = malloc(count * sizeof(addrs[0]));
> +       if (!addrs) {
> +               fprintf(stderr, "Failed to allocate memory for ftrace addresses.\n");
> +               return -1;
> +       }
> +

[...]

>
> +#define SET_SYMBOL(__sym, __var)                                               \
> +       if (!ms->__var && !strcmp(__sym, elf_sym__name(sym, btfe->symtab)))     \
> +               ms->__var = sym->st_value;                                      \
> +
> +static void collect_mcount_symbol(GElf_Sym *sym, struct mcount_symbols *ms)
> +{
> +       if (!ms->start &&
> +           !strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) {
> +               ms->start = sym->st_value;
> +               ms->start_section = sym->st_shndx;
> +       }
> +       SET_SYMBOL("__stop_mcount_loc", stop)
> +       SET_SYMBOL("__init_begin", init_begin)
> +       SET_SYMBOL("__init_end", init_end)

please don't use macro here, it doesn't save much code but complicates
reading it quite significantly

> +}
> +
> +#undef SET_SYMBOL
> +
>  static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
>  {
> +       struct mcount_symbols ms = { };
>         uint32_t core_id;
>         GElf_Sym sym;
>
> @@ -320,6 +485,9 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
>         elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
>                 if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
>                         return -1;
> +               if (collect_function(btfe, &sym))
> +                       return -1;
> +               collect_mcount_symbol(&sym, &ms);
>         }
>
>         if (collect_percpu_vars) {
> @@ -329,9 +497,34 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
>                 if (btf_elf__verbose)
>                         printf("Found %d per-CPU variables!\n", percpu_var_cnt);
>         }
> +
> +       if (functions_cnt) {
> +               qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp);
> +               if (ms.start && ms.stop &&
> +                   filter_functions(btfe, &ms)) {

nit: single line should fit well, no?

> +                       fprintf(stderr, "Failed to filter dwarf functions\n");
> +                       return -1;
> +               }
> +               if (btf_elf__verbose)
> +                       printf("Found %d functions!\n", functions_valid);
> +       }
> +
>         return 0;
>  }
>

[...]

Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa]

On Tue, Nov 03, 2020 at 10:55:58AM -0800, Andrii Nakryiko wrote:

SNIP

> > I can still see several differences to ftrace functions in
> > /sys/kernel/debug/tracing/available_filter_functions file:
> >
> >   - available_filter_functions includes modules (7086 functions)
> >   - available_filter_functions includes functions like:
> >       __acpi_match_device.part.0.constprop.0
> >       acpi_ns_check_sorted_list.constprop.0
> >       acpi_os_unmap_generic_address.part.0
> >       acpiphp_check_bridge.part.0
> >
> >     which are not part of dwarf data (1164 functions)
> >   - BTF includes multiple functions like:
> >       __clk_register_clkdev
> >       clk_register_clkdev
> >
> >     which share same code so they appear just as single function
> >     in available_filter_functions, but dwarf keeps track of both
> >     of them (16 functions)
> >
> > With this change I'm getting 38334 BTF functions, which
> > when added above functions to consideration gives same
> > amount of functions in available_filter_functions.
> >
> > The patch still keeps the original function filter condition
> > (that uses current fn->declaration check) in case the object
> > does not contain *_mcount_loc symbol -> object is not vmlinux.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  btf_encoder.c | 222 +++++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 220 insertions(+), 2 deletions(-)
> 
> [...]
> 
> > +static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
> > +{
> > +       if (elf_sym__type(sym) != STT_FUNC)
> > +               return 0;
> > +       if (!elf_sym__value(sym))
> > +               return 0;
> > +
> > +       if (functions_cnt == functions_alloc) {
> > +               functions_alloc = max(1000, functions_alloc * 3 / 2);
> > +               functions = realloc(functions, functions_alloc * sizeof(*functions));
> > +               if (!functions)
> > +                       return -1;

ok, I thought that if we go down I don't need to,
but I did not check how pahole is handling this,
probably free everything

> 
> memory leak right here. You need to use a temporary variable and check
> if for NULL, before overwriting functions.
> 
> > +       }
> > +
> > +       functions[functions_cnt].name = elf_sym__name(sym, btfe->symtab);
> > +       functions[functions_cnt].addr = elf_sym__value(sym);
> > +       functions[functions_cnt].generated = false;
> > +       functions[functions_cnt].valid = false;
> > +       functions_cnt++;
> > +       return 0;
> > +}
> > +
> > +static int addrs_cmp(const void *_a, const void *_b)
> > +{
> > +       const unsigned long *a = _a;
> > +       const unsigned long *b = _b;
> > +
> > +       return *a - *b;
> 
> this is cute, but is it always correct? instead of thinking how this
> works with overflows, maybe let's keep it simple with
> 
> if (*a == *b)
>   return 0;
> return *a < *b ? -1 : 1;

sure, will fix

> 
> ?
> 
> > +}
> > +
> > +static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
> > +{
> > +       bool init_filter = ms->init_begin && ms->init_end;
> > +       unsigned long *addrs, count, offset, i;
> > +       Elf_Data *data;
> > +       GElf_Shdr shdr;
> > +       Elf_Scn *sec;
> > +
> > +       /*
> > +        * Find mcount addressed marked by __start_mcount_loc
> > +        * and __stop_mcount_loc symbols and load them into
> > +        * sorted array.
> > +        */
> > +       sec = elf_getscn(btfe->elf, ms->start_section);
> > +       if (!sec || !gelf_getshdr(sec, &shdr)) {
> > +               fprintf(stderr, "Failed to get section(%lu) header.\n",
> > +                       ms->start_section);
> > +               return -1;
> > +       }
> > +
> > +       offset = ms->start - shdr.sh_addr;
> > +       count  = (ms->stop - ms->start) / 8;
> > +
> > +       data = elf_getdata(sec, 0);
> > +       if (!data) {
> > +               fprintf(stderr, "Failed to section(%lu) data.\n",
> 
> typo: failed to get?

yep

> 
> > +                       ms->start_section);
> > +               return -1;
> > +       }
> > +
> > +       addrs = malloc(count * sizeof(addrs[0]));
> > +       if (!addrs) {
> > +               fprintf(stderr, "Failed to allocate memory for ftrace addresses.\n");
> > +               return -1;
> > +       }
> > +
> 
> [...]
> 
> >
> > +#define SET_SYMBOL(__sym, __var)                                               \
> > +       if (!ms->__var && !strcmp(__sym, elf_sym__name(sym, btfe->symtab)))     \
> > +               ms->__var = sym->st_value;                                      \
> > +
> > +static void collect_mcount_symbol(GElf_Sym *sym, struct mcount_symbols *ms)
> > +{
> > +       if (!ms->start &&
> > +           !strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) {
> > +               ms->start = sym->st_value;
> > +               ms->start_section = sym->st_shndx;
> > +       }
> > +       SET_SYMBOL("__stop_mcount_loc", stop)
> > +       SET_SYMBOL("__init_begin", init_begin)
> > +       SET_SYMBOL("__init_end", init_end)
> 
> please don't use macro here, it doesn't save much code but complicates
> reading it quite significantly

ok

> 
> > +}
> > +
> > +#undef SET_SYMBOL
> > +
> >  static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> >  {
> > +       struct mcount_symbols ms = { };
> >         uint32_t core_id;
> >         GElf_Sym sym;
> >
> > @@ -320,6 +485,9 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> >         elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
> >                 if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
> >                         return -1;
> > +               if (collect_function(btfe, &sym))
> > +                       return -1;
> > +               collect_mcount_symbol(&sym, &ms);
> >         }
> >
> >         if (collect_percpu_vars) {
> > @@ -329,9 +497,34 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
> >                 if (btf_elf__verbose)
> >                         printf("Found %d per-CPU variables!\n", percpu_var_cnt);
> >         }
> > +
> > +       if (functions_cnt) {
> > +               qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp);
> > +               if (ms.start && ms.stop &&
> > +                   filter_functions(btfe, &ms)) {
> 
> nit: single line should fit well, no?

ook

thanks,
jirka