Hi.
On Thu, 4 Feb 2021 at 04:10, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
>
> 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?
>
Yes. I'll follow up with a trivial patch that does that.
> > 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
> >
