Re: [PATCH] libkmod-elf: resolve CRC if module is built with MODULE_REL_CRCS

[Lucas De Marchi <lucas.de.marchi@gmail.com>]

On Wed, Jul 19, 2017 at 7:56 AM, Yauheni Kaliuta
<yauheni.kaliuta@redhat.com> wrote:
> Normally exported symbol's crc is stored as absolute (SHN_ABS)
> value of special named symbol __crc_<symbol name>.
>
> When the kernel and modules are built with the config option
> CONFIG_MODULE_REL_CRCS, all the CRCs are put in a special section
> and the __crc_<symbol name> symbols values are offsets in the
> section. See patch description of the commit:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=56067812d5b0e737ac2063e94a50f76b810d6ca3
>
> Add kmod support of this configuration.
>
> Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
> ---
>  libkmod/libkmod-elf.c | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/libkmod/libkmod-elf.c b/libkmod/libkmod-elf.c
> index 90da89aebbaf..ef4a8a3142a1 100644
> --- a/libkmod/libkmod-elf.c
> +++ b/libkmod/libkmod-elf.c
> @@ -747,6 +747,31 @@ static inline uint8_t kmod_symbol_bind_from_elf(uint8_t elf_value)
>         }
>  }
>
> +static uint64_t kmod_elf_resolve_crc(const struct kmod_elf *elf, uint64_t crc, uint16_t shndx)
> +{
> +       int err;
> +       uint64_t off, size;
> +       uint32_t nameoff;
> +
> +       if (shndx == SHN_ABS || shndx == SHN_UNDEF)
> +               return crc;
> +
> +       err = elf_get_section_info(elf, shndx, &off, &size, &nameoff);
> +       if (err < 0) {
> +               ELFDBG("Cound not find section index %"PRIu16" for crc", shndx);
> +               return (uint64_t)-1;
> +       }
> +
> +       if (crc > (size - sizeof(uint32_t))) {
> +               ELFDBG("CRC offset %"PRIu64" is too big, section %"PRIu16" size is %"PRIu64"\n",
> +                      crc, shndx, size);
> +               return (uint64_t)-1;
> +       }
> +
> +       crc = elf_get_uint(elf, off + crc, sizeof(uint32_t));
> +       return crc;
> +}
> +
>  /* array will be allocated with strings in a single malloc, just free *array */
>  int kmod_elf_get_symbols(const struct kmod_elf *elf, struct kmod_modversion **array)
>  {
> @@ -830,6 +855,7 @@ int kmod_elf_get_symbols(const struct kmod_elf *elf, struct kmod_modversion **ar
>                 uint32_t name_off;
>                 uint64_t crc;
>                 uint8_t info, bind;
> +               uint16_t shndx;
>
>  #define READV(field)                                                   \
>                 elf_get_uint(elf, sym_off + offsetof(typeof(*s), field),\
> @@ -839,11 +865,13 @@ int kmod_elf_get_symbols(const struct kmod_elf *elf, struct kmod_modversion **ar
>                         name_off = READV(st_name);
>                         crc = READV(st_value);
>                         info = READV(st_info);
> +                       shndx = READV(st_shndx);
>                 } else {
>                         Elf64_Sym *s;
>                         name_off = READV(st_name);
>                         crc = READV(st_value);
>                         info = READV(st_info);
> +                       shndx = READV(st_shndx);
>                 }
>  #undef READV
>                 name = elf_get_mem(elf, str_off + name_off);
> @@ -856,7 +884,7 @@ int kmod_elf_get_symbols(const struct kmod_elf *elf, struct kmod_modversion **ar
>                 else
>                         bind = ELF64_ST_BIND(info);
>
> -               a[count].crc = crc;
> +               a[count].crc = kmod_elf_resolve_crc(elf, crc, shndx);
>                 a[count].bind = kmod_symbol_bind_from_elf(bind);
>                 a[count].symbol = itr;
>                 slen = strlen(name);
> --

LGTM. I'll give this a try and apply.  Do you think it's possible to
build an out-of-tree module with this option so we can add one to the
testsuite?


Lucas De Marchi