[PATCH RFC v3 4/8] tpm: add __always_inline for tpm_is_hwrng_enabled

[Chen Linxuan via B4 Relay <devnull+chenlinxuan.uniontech.com@xxxxxxxxxx>]

From: Winston Wen <wentao@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Signed-off-by: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx>
Signed-off-by: Winston Wen <wentao@xxxxxxxxxxxxx>
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
---
 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;

-- 
2.43.0