Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Feb 10, 2021 at 02:26:43PM +0100, Jiri Olsa wrote:
> On Tue, Feb 09, 2021 at 02:00:29PM -0800, Andrii Nakryiko wrote:
> 
> SNIP
> 
> > > > > I'm still trying to build the kernel.. however ;-)
> > > > >
> > > > > patch below adds the ftrace check only for static functions
> > > > > and lets the externa go through.. but as you said, in this
> > > > > case we'll need to figure out the 'notrace' and other checks
> > > > > ftrace is doing
> > > > >
> > > > > jirka
> > > > >
> > > > >
> > > > > ---
> > > > > diff --git a/btf_encoder.c b/btf_encoder.c
> > > > > index b124ec20a689..4d147406cfa5 100644
> > > > > --- a/btf_encoder.c
> > > > > +++ b/btf_encoder.c
> > > > > @@ -734,7 +734,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> > > > >                         continue;
> > > > >                 if (!has_arg_names(cu, &fn->proto))
> > > > >                         continue;
> > > > > -               if (functions_cnt) {
> > > > > +               if (!fn->external && functions_cnt) {
> > > >
> > > > I wouldn't trust DWARF, honestly. Wouldn't checking GLOBAL vs LOCAL
> > > > FUNC ELF symbol be more reliable?
> > >
> > > that'd mean extra bsearch on each processed function,
> > > on the ther hand, we'are already slow ;-) I'll check
> > > how big the slowdown would be
> > >
> > 
> > We currently record addresses and do binary search. Now we need to
> > record address + size and still do binary search with a slightly
> > different semantics (find closest entry >= addr). Then just check that
> > it overlaps, taking into account the length of the function code. It
> > shouldn't result in a noticeable slowdown. Might be actually faster,
> > because we might avoid callback function call costs.
> 
> I'm still not sure how to handle the external check for function via elf,
> but below is change for checking that ftrace addrs are within elf functions
> 
> seems to work in my tests, I'll run some more tests and send full patch
> 
> jirka

This passes the build test with the initial problematic configuration
file and the reduced one that I came up with. That is about the extent
to which I can verify that it works though :)

> ---
> diff --git a/btf_encoder.c b/btf_encoder.c
> index b124ec20a689..548a12847f99 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -36,6 +36,7 @@ struct funcs_layout {
>  struct elf_function {
>  	const char	*name;
>  	unsigned long	 addr;
> +	unsigned long	 end;
>  	unsigned long	 sh_addr;
>  	bool		 generated;
>  };
> @@ -44,7 +45,7 @@ static struct elf_function *functions;
>  static int functions_alloc;
>  static int functions_cnt;
>  
> -static int functions_cmp(const void *_a, const void *_b)
> +static int functions_cmp_name(const void *_a, const void *_b)
>  {
>  	const struct elf_function *a = _a;
>  	const struct elf_function *b = _b;
> @@ -52,6 +53,16 @@ static int functions_cmp(const void *_a, const void *_b)
>  	return strcmp(a->name, b->name);
>  }
>  
> +static int functions_cmp_addr(const void *_a, const void *_b)
> +{
> +	const struct elf_function *a = _a;
> +	const struct elf_function *b = _b;
> +
> +	if (a->addr == b->addr)
> +		return 0;
> +	return a->addr < b->addr ? -1 : 1;
> +}
> +
>  static void delete_functions(void)
>  {
>  	free(functions);
> @@ -98,6 +109,7 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
>  
>  	functions[functions_cnt].name = name;
>  	functions[functions_cnt].addr = elf_sym__value(sym);
> +	functions[functions_cnt].end = (__u64) -1;
>  	functions[functions_cnt].sh_addr = sh.sh_addr;
>  	functions[functions_cnt].generated = false;
>  	functions_cnt++;
> @@ -236,9 +248,25 @@ get_kmod_addrs(struct btf_elf *btfe, __u64 **paddrs, __u64 *pcount)
>  	return 0;
>  }
>  
> +static bool is_addr_in_func(__u64 addr, struct elf_function *func, bool kmod)
> +{
> +	/*
> +	 * For vmlinux image both addrs[x] and functions[x]::addr
> +	 * values are final address and are comparable.
> +	 *
> +	 * For kernel module addrs[x] is final address, but
> +	 * functions[x]::addr is relative address within section
> +	 * and needs to be relocated by adding sh_addr.
> +	 */
> +	__u64 start = kmod ? func->addr + func->sh_addr : func->addr;
> +	__u64 end = kmod ? func->end+ func->sh_addr : func->end;
> +
> +	return start <= addr && addr < end;
> +}
> +
>  static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
>  {
> -	__u64 *addrs, count, i;
> +	__u64 *addrs, count, i_func, i_addr;
>  	int functions_valid = 0;
>  	bool kmod = false;
>  
> @@ -266,43 +294,62 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
>  		return 0;
>  	}
>  
> -	qsort(addrs, count, sizeof(addrs[0]), addrs_cmp);
> -	qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp);
> -
>  	/*
> -	 * Let's got through all collected functions and filter
> -	 * out those that are not in ftrace.
> +	 * Sort both functions and addrs so we can iterate
> +	 * both of them simultaneously and found matching
> +	 * func/addr pairs.
>  	 */
> -	for (i = 0; i < functions_cnt; i++) {
> -		struct elf_function *func = &functions[i];
> -		/*
> -		 * For vmlinux image both addrs[x] and functions[x]::addr
> -		 * values are final address and are comparable.
> -		 *
> -		 * For kernel module addrs[x] is final address, but
> -		 * functions[x]::addr is relative address within section
> -		 * and needs to be relocated by adding sh_addr.
> -		 */
> -		__u64 addr = kmod ? func->addr + func->sh_addr : func->addr;
> +	qsort(addrs, count, sizeof(addrs[0]), addrs_cmp);
> +	qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp_addr);
> +
> +	for (i_func = 0, i_addr = 0; i_func < functions_cnt; i_func++) {
> +		struct elf_function *func = &functions[i_func];
> +
> +		if (i_func + 1 < functions_cnt)
> +			func->end = functions[i_func + 1].addr;
> +
> +		for (; i_addr < count; i_addr++) {
> +			__u64 addr = addrs[i_addr];
> +
> +			/* Functions are  ahead, catch up with addrs. */
> +			if (addr < func->addr)
> +				continue;
> +
> +			/* Addr is within function - mark function as valid. */
> +			if (is_addr_in_func(addr, func, kmod)) {
> +				/*
> +				 * We iterate over sorted array, so we can easily skip
> +				 * not valid item and move following valid field into
> +				 * its place, and still keep the 'new' array sorted.
> +				 */
> +				if (i_func != functions_valid)
> +					functions[functions_valid] = functions[i_func];
> +				functions_valid++;
> +				i_addr++;
> +			}
>  
> -		/* Make sure function is within ftrace addresses. */
> -		if (bsearch(&addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
>  			/*
> -			 * We iterate over sorted array, so we can easily skip
> -			 * not valid item and move following valid field into
> -			 * its place, and still keep the 'new' array sorted.
> +			 * Addrs are ahead, catch up with functions, or we just
> +			 * found valid function and want to move to another.
>  			 */
> -			if (i != functions_valid)
> -				functions[functions_valid] = functions[i];
> -			functions_valid++;
> +			break;
>  		}
>  	}
>  
> +	if (btf_elf__verbose) {
> +		printf("Found %d functions out of %d symbols and %llu ftrace addresses.\n",
> +			functions_valid, functions_cnt, count);
> +	}
> +
>  	functions_cnt = functions_valid;
>  	free(addrs);
>  
> -	if (btf_elf__verbose)
> -		printf("Found %d functions!\n", functions_cnt);
> +	/*
> +	 * And finaly sort 'valid' functions by name,
> +	 * so find_function can be used.
> +	 */
> +	qsort(functions, functions_cnt, sizeof(functions[0]), functions_cmp_name);
> +
>  	return 0;
>  }
>  
> @@ -312,7 +359,7 @@ static struct elf_function *find_function(const struct btf_elf *btfe,
>  	struct elf_function key = { .name = name };
>  
>  	return bsearch(&key, functions, functions_cnt, sizeof(functions[0]),
> -		       functions_cmp);
> +		       functions_cmp_name);
>  }
>  
>  static bool btf_name_char_ok(char c, bool first)
>