[RFC,v2,4/5] tpm: add __always_inline for tpm_is_hwrng_enabled

Commit Message

Chen Linxuan April 16, 2025, 9:44 a.m. UTC

From: Winston Wen <[e-mail]>

Presume that kernel is compiled for x86_64 with gcc version 13.3.0:

  make defconfig
  ./scripts/kconfig/merge_config.sh .config <(
    echo CONFIG_TCG_TPM=y
    echo CONFIG_HW_RANDOM=m
  )
  make KCFLAGS="-fno-inline-small-functions -fno-inline-functions-called-once"

This results a link error:

  ld: vmlinux.o: in function `tpm_add_hwrng':
  tpm-chip.c:(.text+0x6c5924): undefined reference to `hwrng_register'
  ld: vmlinux.o: in function `tpm_chip_unregister':
  (.text+0x6c5bc9): undefined reference to `hwrng_unregister'
  ld: vmlinux.o: in function `tpm_chip_register':
  (.text+0x6c5c9b): undefined reference to `hwrng_unregister'

With `CONFIG_TCG_TPM=y` and `CONFIG_HW_RANDOM=m`,
the functions `tpm_add_hwrng`, `tpm_chip_unregister`, and
`tpm_chip_register` are compiled into `vmlinux.o`
and reference the symbols `hwrng_register` and `hwrng_unregister`.
These symbols, however, are compiled into `rng-core.ko`, which results
in the linking error.

I am not sure but I think this weird linking error only arises when
auto inlining is disabled because of some dead code elimination.

`CONFIG_TCG_TPM=y` and `CONFIG_HW_RANDOM=m` set `CONFIG_HW_RANDOM_TPM=n`.
This causes the function `tpm_is_hwrng_enabled` to always return
`false`, as shown below:

  static bool tpm_is_hwrng_enabled(struct tpm_chip *chip)
  {
      if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM))
          return false;
      if (tpm_is_firmware_upgrade(chip))
          return false;
      if (chip->flags & TPM_CHIP_FLAG_HWRNG_DISABLED)
          return false;
      return true;
  }

When `tpm_is_hwrng_enabled` is inlined, dead code elimination
optimizations are applied and the reference to the `hwrng_*` functions
will been removed.
For instance, in the `tpm_chip_unregister` function:

  void tpm_chip_unregister(struct tpm_chip *chip)
  {
  #ifdef CONFIG_TCG_TPM2_HMAC
      int rc;

      rc = tpm_try_get_ops(chip);
      if (!rc) {
          tpm2_end_auth_session(chip);
          tpm_put_ops(chip);
      }
  #endif

      tpm_del_legacy_sysfs(chip);
      if (tpm_is_hwrng_enabled(chip))
          hwrng_unregister(&chip->hwrng);
      tpm_bios_log_teardown(chip);
      if (chip->flags & TPM_CHIP_FLAG_TPM2 && !tpm_is_firmware_upgrade(chip))
          tpm_devs_remove(chip);
      tpm_del_char_device(chip);
  }

When `tpm_is_hwrng_enabled` is inlined and always returns `false`,
the call to `hwrng_unregister` is effectively part of a `if (false)`
block, which I guess that will be then optimized out.

However, when the `-fno-inline-small-functions` and
`-fno-inline-functions-called-once` flags are used,
tpm_is_hwrng_enabled is not inline.

And this optimization some how cannot occur,
leading to the undefined reference errors during linking.

Adding the `__always_inline` attribute ensures that
`tpm_is_hwrng_enabled` is inlined regardless of the compiler flags.
This allows the dead code elimination to proceed as expected,
resolving the linking issue.

Co-developed-by: Chen Linxuan <[e-mail]>
Signed-off-by: Chen Linxuan <[e-mail]>
Signed-off-by: Winston Wen <[e-mail]>
---
 drivers/char/tpm/tpm-chip.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Jarkko Sakkinen April 16, 2025, 7:34 p.m. UTC | #1

On Wed, Apr 16, 2025 at 05:44:50PM +0800, Chen Linxuan via B4 Relay wrote:
> From: Winston Wen <[e-mail]>
> 
> Presume that kernel is compiled for x86_64 with gcc version 13.3.0:
> 
>   make defconfig
>   ./scripts/kconfig/merge_config.sh .config <(
>     echo CONFIG_TCG_TPM=y
>     echo CONFIG_HW_RANDOM=m
>   )
>   make KCFLAGS="-fno-inline-small-functions -fno-inline-functions-called-once"
> 
> This results a link error:
> 
>   ld: vmlinux.o: in function `tpm_add_hwrng':
>   tpm-chip.c:(.text+0x6c5924): undefined reference to `hwrng_register'
>   ld: vmlinux.o: in function `tpm_chip_unregister':
>   (.text+0x6c5bc9): undefined reference to `hwrng_unregister'
>   ld: vmlinux.o: in function `tpm_chip_register':
>   (.text+0x6c5c9b): undefined reference to `hwrng_unregister'
> 
> With `CONFIG_TCG_TPM=y` and `CONFIG_HW_RANDOM=m`,
> the functions `tpm_add_hwrng`, `tpm_chip_unregister`, and
> `tpm_chip_register` are compiled into `vmlinux.o`
> and reference the symbols `hwrng_register` and `hwrng_unregister`.
> These symbols, however, are compiled into `rng-core.ko`, which results
> in the linking error.
> 
> I am not sure but I think this weird linking error only arises when
> auto inlining is disabled because of some dead code elimination.
> 
> `CONFIG_TCG_TPM=y` and `CONFIG_HW_RANDOM=m` set `CONFIG_HW_RANDOM_TPM=n`.
> This causes the function `tpm_is_hwrng_enabled` to always return
> `false`, as shown below:
> 
>   static bool tpm_is_hwrng_enabled(struct tpm_chip *chip)
>   {
>       if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM))
>           return false;
>       if (tpm_is_firmware_upgrade(chip))
>           return false;
>       if (chip->flags & TPM_CHIP_FLAG_HWRNG_DISABLED)
>           return false;
>       return true;
>   }
> 
> When `tpm_is_hwrng_enabled` is inlined, dead code elimination
> optimizations are applied and the reference to the `hwrng_*` functions
> will been removed.
> For instance, in the `tpm_chip_unregister` function:
> 
>   void tpm_chip_unregister(struct tpm_chip *chip)
>   {
>   #ifdef CONFIG_TCG_TPM2_HMAC
>       int rc;
> 
>       rc = tpm_try_get_ops(chip);
>       if (!rc) {
>           tpm2_end_auth_session(chip);
>           tpm_put_ops(chip);
>       }
>   #endif
> 
>       tpm_del_legacy_sysfs(chip);
>       if (tpm_is_hwrng_enabled(chip))
>           hwrng_unregister(&chip->hwrng);
>       tpm_bios_log_teardown(chip);
>       if (chip->flags & TPM_CHIP_FLAG_TPM2 && !tpm_is_firmware_upgrade(chip))
>           tpm_devs_remove(chip);
>       tpm_del_char_device(chip);
>   }
> 
> When `tpm_is_hwrng_enabled` is inlined and always returns `false`,
> the call to `hwrng_unregister` is effectively part of a `if (false)`
> block, which I guess that will be then optimized out.
> 
> However, when the `-fno-inline-small-functions` and
> `-fno-inline-functions-called-once` flags are used,
> tpm_is_hwrng_enabled is not inline.
> 
> And this optimization some how cannot occur,
> leading to the undefined reference errors during linking.
> 
> Adding the `__always_inline` attribute ensures that
> `tpm_is_hwrng_enabled` is inlined regardless of the compiler flags.
> This allows the dead code elimination to proceed as expected,
> resolving the linking issue.
> 
> Co-developed-by: Chen Linxuan <[e-mail]>
> Signed-off-by: Chen Linxuan <[e-mail]>
> Signed-off-by: Winston Wen <[e-mail]>
> ---
>  drivers/char/tpm/tpm-chip.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index e25daf2396d37bcaeae8a96267764df0861ad1be..48cc74d84247e258a39f2118e03aa10d0cbb066a 100644
> --- a/drivers/char/tpm/tpm-chip.c
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -534,7 +534,7 @@ static int tpm_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)
>  	return tpm_get_random(chip, data, max);
>  }
>  
> -static bool tpm_is_hwrng_enabled(struct tpm_chip *chip)
> +static __always_inline bool tpm_is_hwrng_enabled(struct tpm_chip *chip)
>  {
>  	if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM))
>  		return false;
> 
> -- 
> 2.48.1
> 
> 
> 

Thank you.

Reviewed-by: Jarkko Sakkinen <[e-mail]>


BR, Jarkko

Patch

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index e25daf2396d37bcaeae8a96267764df0861ad1be..48cc74d84247e258a39f2118e03aa10d0cbb066a 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -534,7 +534,7 @@  static int tpm_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 	return tpm_get_random(chip, data, max);
 }
 
-static bool tpm_is_hwrng_enabled(struct tpm_chip *chip)
+static __always_inline bool tpm_is_hwrng_enabled(struct tpm_chip *chip)
 {
 	if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM))
 		return false;