Re: [PATCH dwarves v2 4/4] btf_encoder: Align .BTF section/segment to 8 bytes

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Giuliano Procida <gprocida@google.com>
Cc: dwarves@vger.kernel.org,
	"Arnaldo Carvalho de Melo" <acme@kernel.org>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Matthias Männich" <maennich@google.com>,
	kernel-team@android.com, "Kernel Team" <kernel-team@fb.com>,
	bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH dwarves v2 4/4] btf_encoder: Align .BTF section/segment to 8 bytes
Date: Wed, 3 Feb 2021 20:10:03 -0800	[thread overview]
Message-ID: <CAEf4Bzb6bKVo3NeVUr3Zb9+is0XbeOn+xLXtHv-GA_cWGVZfnA@mail.gmail.com> (raw)
In-Reply-To: <20210201172530.1141087-5-gprocida@google.com>

On Mon, Feb 1, 2021 at 9:26 AM Giuliano Procida <gprocida@google.com> wrote:
>
> This is to avoid misaligned access to BTF type structs when
> memory-mapping ELF sections.
>
> Signed-off-by: Giuliano Procida <gprocida@google.com>
> ---
>  libbtf.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/libbtf.c b/libbtf.c
> index 048a873..ae99a93 100644
> --- a/libbtf.c
> +++ b/libbtf.c
> @@ -755,7 +755,13 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>          * This actually happens in practice with vmlinux which has .strtab
>          * after .shstrtab, resulting in a (small) hole the size of the original
>          * .shstrtab.
> +        *
> +        * We'll align .BTF to 8 bytes to cater for all architectures. It'd be
> +        * nice if we could fetch this value from somewhere. The BTF
> +        * specification does not discuss alignment and its trailing string
> +        * table is not currently padded to any particular alignment.
>          */
> +       const size_t btf_alignment = 8;
>
>         /*
>          * First we look if there was already a .BTF section present and
> @@ -847,8 +853,8 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>         elf_flagdata(btf_data, ELF_C_SET, ELF_F_DIRTY);
>
>         /* Update .BTF section in the SHT */
> -       size_t new_btf_offset = high_water_mark;
> -       size_t new_btf_size = raw_btf_size;
> +       size_t new_btf_offset = roundup(high_water_mark, btf_alignment);
> +       size_t new_btf_size = roundup(raw_btf_size, btf_alignment);
>         GElf_Shdr btf_shdr_mem;
>         GElf_Shdr *btf_shdr = gelf_getshdr(btf_scn, &btf_shdr_mem);
>         if (!btf_shdr) {
> @@ -856,6 +862,7 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>                         __func__, elf_errmsg(elf_errno()));
>                 goto out;
>         }
> +       btf_shdr->sh_addralign = btf_alignment;

if we set just this and let libelf do the layout, would libelf ensure
8-byte alignment of .BTF section inside the ELF file?

>         btf_shdr->sh_entsize = 0;
>         btf_shdr->sh_flags = SHF_ALLOC;
>         if (dot_btf_offset)
> @@ -926,6 +933,7 @@ static int btf_elf__write(const char *filename, struct btf *btf)
>                 pht[phnum].p_memsz = pht[phnum].p_filesz = btf_shdr->sh_size;
>                 pht[phnum].p_vaddr = pht[phnum].p_paddr = 0;
>                 pht[phnum].p_flags = PF_R;
> +               pht[phnum].p_align = btf_alignment;
>                 void *phdr = gelf_newphdr(elf, phnum+1);
>                 if (!phdr) {
>                         fprintf(stderr, "%s: gelf_newphdr failed: %s\n",
> --
> 2.30.0.365.g02bc693789-goog
>