On Mon, Feb 1, 2021 at 9:26 AM Giuliano Procida <gprocida@xxxxxxxxxx> wrote:
>
> This is to avoid misaligned access to BTF type structs when
> memory-mapping ELF sections.
>
> Signed-off-by: Giuliano Procida <gprocida@xxxxxxxxxx>
> ---
> 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
>
