libkmod-elf: resolve CRC if module is built with MODULE_REL_CRCS
diff mbox

Message ID 20170719145649.20334-1-yauheni.kaliuta@redhat.com
State New
Headers show

Commit Message

Yauheni Kaliuta July 19, 2017, 2:56 p.m. UTC
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(-)

Comments

Lucas De Marchi July 19, 2017, 5:51 p.m. UTC | #1
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
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Yauheni Kaliuta July 19, 2017, 7:04 p.m. UTC | #2
Hi, Lucas!

>>>>> On Wed, 19 Jul 2017 10:51:44 -0700, Lucas De Marchi  wrote:
 > 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.

[...]

 > LGTM. I'll give this a try and apply.

Thanks! I would also like to get some feedback from the kernel feature
author, Ard Biesheuvel, that this does not miss other usecases.

 > Do you think it's possible to build an out-of-tree module with this
 > option so we can add one to the testsuite?

I'll check what I can do.

The problem is seen if you run depmod with -e -E, which I guess is not used
too often.
Ard Biesheuvel July 19, 2017, 7:10 p.m. UTC | #3
On 19 July 2017 at 20:04, Yauheni Kaliuta <yauheni.kaliuta@redhat.com> wrote:
> Hi, Lucas!
>
>>>>>> On Wed, 19 Jul 2017 10:51:44 -0700, Lucas De Marchi  wrote:
>  > 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.
>
> [...]
>
>  > LGTM. I'll give this a try and apply.
>
> Thanks! I would also like to get some feedback from the kernel feature
> author, Ard Biesheuvel, that this does not miss other usecases.
>

Hi all,

The patch looks correct to me. My only comment is that
kmod_elf_resolve_crc() should probably return a uint32_t instead,
given the size of a CRC32.

In any case,

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Thanks,
Ard.


>  > Do you think it's possible to build an out-of-tree module with this
>  > option so we can add one to the testsuite?
>
> I'll check what I can do.
>
> The problem is seen if you run depmod with -e -E, which I guess is not used
> too often.
>
> --
> WBR,
> Yauheni Kaliuta
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Yauheni Kaliuta July 19, 2017, 7:33 p.m. UTC | #4
Hi, Ard!

>>>>> On Wed, 19 Jul 2017 20:10:17 +0100, Ard Biesheuvel  wrote:

 > On 19 July 2017 at 20:04, Yauheni Kaliuta <yauheni.kaliuta@redhat.com> wrote:
 >> Hi, Lucas!
 >> 
 >>>>>>> On Wed, 19 Jul 2017 10:51:44 -0700, Lucas De Marchi  wrote:
 >> > 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.
 >> 
 >> [...]
 >> 
 >> > LGTM. I'll give this a try and apply.
 >> 
 >> Thanks! I would also like to get some feedback from the kernel feature
 >> author, Ard Biesheuvel, that this does not miss other usecases.
 >> 

 > The patch looks correct to me. My only comment is that
 > kmod_elf_resolve_crc() should probably return a uint32_t instead,
 > given the size of a CRC32.

Well, it's a specific of the internal libkmod implementation. Both
struct kmod_modversion::crc and return value of elf_get_uint()
are uint64_t, but in the patch elf_get_uint() reads correct
sizeof(uint32_t) value from the ELF.

 > In any case,

 > Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Thanks a lot!

Patch
diff mbox

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);