Re: [PATCH 1/2] elf_symtab: Add support for SHN_XINDEX index to elf_section_by_name

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Jiri Olsa <jolsa@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andriin@fb.com>,
	dwarves@vger.kernel.org, netdev@vger.kernel.org,
	bpf@vger.kernel.org, Yonghong Song <yhs@fb.com>,
	Hao Luo <haoluo@google.com>, Martin KaFai Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Joe Lawrence <joe.lawrence@redhat.com>,
	Mark Wielaard <mjw@redhat.com>
Subject: Re: [PATCH 1/2] elf_symtab: Add support for SHN_XINDEX index to elf_section_by_name
Date: Fri, 22 Jan 2021 16:45:59 -0300	[thread overview]
Message-ID: <20210122194559.GA617095@kernel.org> (raw)
In-Reply-To: <20210122163920.59177-2-jolsa@kernel.org>

Em Fri, Jan 22, 2021 at 05:39:19PM +0100, Jiri Olsa escreveu:
> In case the elf's header e_shstrndx contains SHN_XINDEX,
> we need to call elf_getshdrstrndx to get the proper
> string table index.

Applied, but changed the changelog comment to:

------------------------------------------------------------------
elf_symtab: Handle SHN_XINDEX index in elf_section_by_name()

Use elf_getshdrstrndx() to cover the case where the ELF header
'e_shstrndx' field contains the special value SHN_XINDEX so that we get
the proper string table index.

This is necessary to handle files with over 65536 sections, such as when
building the kernel with -f[function|data]-sections.  Other cases may
include when using FG-ASLR, LTO.

With so many sections, ELF is using extended section index table, which
is used to hold values for some of the indexes and extra code is needed
to retrieve them.
------------------------------------------------------------------

This is from the thread, so that we can have a more comprehensive idea
of what is this SHN_XINDEX and where it can show up when looking at this
code 10 years from now (or next month) :-)

Holler if I've messed up something.

Thanks,

- Arnaldo

> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  dutil.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/dutil.c b/dutil.c
> index 7b667647420f..11fb7202049c 100644
> --- a/dutil.c
> +++ b/dutil.c
> @@ -179,12 +179,18 @@ Elf_Scn *elf_section_by_name(Elf *elf, GElf_Ehdr *ep,
>  {
>  	Elf_Scn *sec = NULL;
>  	size_t cnt = 1;
> +	size_t str_idx;
> +
> +	if (elf_getshdrstrndx(elf, &str_idx))
> +		return NULL;
>  
>  	while ((sec = elf_nextscn(elf, sec)) != NULL) {
>  		char *str;
>  
>  		gelf_getshdr(sec, shp);
> -		str = elf_strptr(elf, ep->e_shstrndx, shp->sh_name);
> +		str = elf_strptr(elf, str_idx, shp->sh_name);
> +		if (!str)
> +			return NULL;
>  		if (!strcmp(name, str)) {
>  			if (index)
>  				*index = cnt;
> -- 
> 2.26.2
> 

-- 

- Arnaldo