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

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, Apr 16, 2025 at 05:44:50PM +0800, Chen Linxuan via B4 Relay wrote:
> From: Winston Wen <wentao@uniontech.com>
> 
> 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 <chenlinxuan@uniontech.com>
> Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
> Signed-off-by: Winston Wen <wentao@uniontech.com>
> ---
>  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 <jarkko@kernel.org>


BR, Jarkko