Re: [PATCH] recordmcount: support >64k sections

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 22 Apr 2020 16:24:17 -0700
Sami Tolvanen <samitolvanen@google.com> wrote:

> When compiling a kernel with Clang and LTO, we need to run
> recordmcount on vmlinux.o with a large number of sections, which
> currently fails as the program doesn't understand extended
> section indexes. This change adds support for processing binaries
> with >64k sections.

Thanks! As you have also Cc'd Matt Helsley, perhaps you have noticed we
are trying to get this code merged with objtool.

How would that affect this?

-- Steve



> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
>  scripts/recordmcount.h | 75
> ++++++++++++++++++++++++++++++++++++++---- 1 file changed, 69
> insertions(+), 6 deletions(-)
> 
> diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
> index 74eab03e31d4..b48163864cca 100644
> --- a/scripts/recordmcount.h
> +++ b/scripts/recordmcount.h
> @@ -29,6 +29,9 @@
>  #undef has_rel_mcount
>  #undef tot_relsize
>  #undef get_mcountsym
> +#undef get_shnum
> +#undef get_shstrndx
> +#undef get_symindex
>  #undef get_sym_str_and_relp
>  #undef do_func
>  #undef Elf_Addr
> @@ -58,6 +61,9 @@
>  # define __has_rel_mcount	__has64_rel_mcount
>  # define has_rel_mcount		has64_rel_mcount
>  # define tot_relsize		tot64_relsize
> +# define get_shnum		get_shnum64
> +# define get_shstrndx		get_shstrndx64
> +# define get_symindex		get_symindex64
>  # define get_sym_str_and_relp	get_sym_str_and_relp_64
>  # define do_func		do64
>  # define get_mcountsym		get_mcountsym_64
> @@ -91,6 +97,9 @@
>  # define __has_rel_mcount	__has32_rel_mcount
>  # define has_rel_mcount		has32_rel_mcount
>  # define tot_relsize		tot32_relsize
> +# define get_shnum		get_shnum32
> +# define get_shstrndx		get_shstrndx32
> +# define get_symindex		get_symindex32
>  # define get_sym_str_and_relp	get_sym_str_and_relp_32
>  # define do_func		do32
>  # define get_mcountsym		get_mcountsym_32
> @@ -173,6 +182,38 @@ static int MIPS_is_fake_mcount(Elf_Rel const *rp)
>  	return is_fake;
>  }
>  
> +static unsigned int get_symindex(Elf_Sym const *sym, Elf32_Word
> const *symtab,
> +				 Elf32_Word const *symtab_shndx)
> +{
> +	unsigned long offset;
> +	int index;
> +
> +	if (sym->st_shndx != SHN_XINDEX)
> +		return w2(sym->st_shndx);
> +
> +	offset = (unsigned long)sym - (unsigned long)symtab;
> +	index = offset / sizeof(*sym);
> +
> +	return w(symtab_shndx[index]);
> +}
> +
> +static unsigned int get_shnum(Elf_Ehdr const *ehdr, Elf_Shdr const
> *shdr0) +{
> +	if (shdr0 && !ehdr->e_shnum)
> +		return w(shdr0->sh_size);
> +
> +	return w2(ehdr->e_shnum);
> +}
> +
> +static int get_shstrndx(Elf_Ehdr const *ehdr, Elf_Shdr const *shdr0)
> +{
> +	if (ehdr->e_shstrndx != SHN_XINDEX)
> +		return w2(ehdr->e_shstrndx);
> +
> +	return w(shdr0->sh_link);
> +}
> +
> +
>  /* Append the new shstrtab, Elf_Shdr[], __mcount_loc and its
> relocations. */ static int append_func(Elf_Ehdr *const ehdr,
>  			Elf_Shdr *const shstr,
> @@ -188,10 +229,12 @@ static int append_func(Elf_Ehdr *const ehdr,
>  	char const *mc_name = (sizeof(Elf_Rela) == rel_entsize)
>  		? ".rela__mcount_loc"
>  		:  ".rel__mcount_loc";
> -	unsigned const old_shnum = w2(ehdr->e_shnum);
>  	uint_t const old_shoff = _w(ehdr->e_shoff);
>  	uint_t const old_shstr_sh_size   = _w(shstr->sh_size);
>  	uint_t const old_shstr_sh_offset = _w(shstr->sh_offset);
> +	Elf_Shdr *const shdr0 = (Elf_Shdr *)(old_shoff + (void
> *)ehdr);
> +	unsigned const old_shnum = get_shnum(ehdr, shdr0);
> +	unsigned const new_shnum = 2 + old_shnum; /*
> {.rel,}__mcount_loc */ uint_t t = 1 + strlen(mc_name) +
> _w(shstr->sh_size); uint_t new_e_shoff;
>  
> @@ -201,6 +244,12 @@ static int append_func(Elf_Ehdr *const ehdr,
>  	t += (_align & -t);  /* word-byte align */
>  	new_e_shoff = t;
>  
> +	if (!ehdr->e_shnum || new_shnum >= SHN_LORESERVE) {
> +		ehdr->e_shnum = 0;
> +		shdr0->sh_size = w(new_shnum);
> +	} else
> +		ehdr->e_shnum = w2(2 + w2(ehdr->e_shnum));
> +
>  	/* body for new shstrtab */
>  	if (ulseek(sb.st_size, SEEK_SET) < 0)
>  		return -1;
> @@ -255,7 +304,6 @@ static int append_func(Elf_Ehdr *const ehdr,
>  		return -1;
>  
>  	ehdr->e_shoff = _w(new_e_shoff);
> -	ehdr->e_shnum = w2(2 + w2(ehdr->e_shnum));  /*
> {.rel,}__mcount_loc */ if (ulseek(0, SEEK_SET) < 0)
>  		return -1;
>  	if (uwrite(ehdr, sizeof(*ehdr)) < 0)
> @@ -434,6 +482,8 @@ static int find_secsym_ndx(unsigned const txtndx,
>  				uint_t *const recvalp,
>  				unsigned int *sym_index,
>  				Elf_Shdr const *const symhdr,
> +				Elf32_Word const *symtab,
> +				Elf32_Word const *symtab_shndx,
>  				Elf_Ehdr const *const ehdr)
>  {
>  	Elf_Sym const *const sym0 = (Elf_Sym const
> *)(_w(symhdr->sh_offset) @@ -445,7 +495,7 @@ static int
> find_secsym_ndx(unsigned const txtndx, for (symp = sym0, t = nsym; t;
> --t, ++symp) { unsigned int const st_bind =
> ELF_ST_BIND(symp->st_info); 
> -		if (txtndx == w2(symp->st_shndx)
> +		if (txtndx == get_symindex(symp, symtab,
> symtab_shndx) /* avoid STB_WEAK */
>  		    && (STB_LOCAL == st_bind || STB_GLOBAL ==
> st_bind)) { /* function symbols on ARM have quirks, avoid them */
> @@ -516,21 +566,23 @@ static unsigned tot_relsize(Elf_Shdr const
> *const shdr0, return totrelsz;
>  }
>  
> -
>  /* Overall supervision for Elf32 ET_REL file. */
>  static int do_func(Elf_Ehdr *const ehdr, char const *const fname,
>  		   unsigned const reltype)
>  {
>  	Elf_Shdr *const shdr0 = (Elf_Shdr *)(_w(ehdr->e_shoff)
>  		+ (void *)ehdr);
> -	unsigned const nhdr = w2(ehdr->e_shnum);
> -	Elf_Shdr *const shstr = &shdr0[w2(ehdr->e_shstrndx)];
> +	unsigned const nhdr = get_shnum(ehdr, shdr0);
> +	Elf_Shdr *const shstr = &shdr0[get_shstrndx(ehdr, shdr0)];
>  	char const *const shstrtab = (char const
> *)(_w(shstr->sh_offset)
>  		+ (void *)ehdr);
>  
>  	Elf_Shdr const *relhdr;
>  	unsigned k;
>  
> +	Elf32_Word *symtab = NULL;
> +	Elf32_Word *symtab_shndx = NULL;
> +
>  	/* Upper bound on space: assume all relevant relocs are for
> mcount. */ unsigned       totrelsz;
>  
> @@ -561,6 +613,16 @@ static int do_func(Elf_Ehdr *const ehdr, char
> const *const fname, return -1;
>  	}
>  
> +	for (relhdr = shdr0, k = nhdr; k; --k, ++relhdr) {
> +		if (relhdr->sh_type == SHT_SYMTAB)
> +			symtab = (void *)ehdr + relhdr->sh_offset;
> +		else if (relhdr->sh_type == SHT_SYMTAB_SHNDX)
> +			symtab_shndx = (void *)ehdr +
> relhdr->sh_offset; +
> +		if (symtab && symtab_shndx)
> +			break;
> +	}
> +
>  	for (relhdr = shdr0, k = nhdr; k; --k, ++relhdr) {
>  		char const *const txtname = has_rel_mcount(relhdr,
> shdr0, shstrtab, fname);
> @@ -577,6 +639,7 @@ static int do_func(Elf_Ehdr *const ehdr, char
> const *const fname, result = find_secsym_ndx(w(relhdr->sh_info),
> txtname, &recval, &recsym,
>  						&shdr0[symsec_sh_link],
> +						symtab, symtab_shndx,
>  						ehdr);
>  			if (result)
>  				goto out;
> 
> base-commit: c578ddb39e565139897124e74e5a43e56538cb33