[PATCH 2/2] powerpc/text-patching: Fix possible stringop-overread compilation error

[Xie Yuanbin <xieyuanbin1@huawei.com>]

For strnlen(), if the compiler detects that the maxlen argument exceeds
the valid memory size of the input string object, a compilation error may
occur.

For lastest linux-next source, changing ppc_kallsyms_lookup_name() to
__always_inline, using default ppc64_defconfig, and setting
CONFIG_EXPERT=y, CONFIG_PPC64_BIG_ENDIAN_ELF_ABI_V2=n,
CONFIG_CC_OPTIMIZE_FOR_SIZE=y. Then, when using gcc-15 for compilation,
the following error will be triggered:
```log
  CC      arch/powerpc/kernel/optprobes.o
In file included from ./arch/powerpc/include/asm/kprobes.h:24,
                 from ./include/linux/kprobes.h:31,
                 from arch/powerpc/kernel/optprobes.c:8:
In function ‘ppc_kallsyms_lookup_name’,
    inlined from ‘arch_prepare_optimized_kprobe’ at arch/powerpc/kernel/optprobes.c:209:21:
./arch/powerpc/include/asm/text-patching.h:232:13: error: ‘strnlen’ specified bound 512 exceeds source size 19 [-Werror=stringop-overread]
  232 |         if (strnlen(name, KSYM_NAME_LEN) >= KSYM_NAME_LEN)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘ppc_kallsyms_lookup_name’,
    inlined from ‘arch_prepare_optimized_kprobe’ at arch/powerpc/kernel/optprobes.c:210:22:
./arch/powerpc/include/asm/text-patching.h:232:13: error: ‘strnlen’ specified bound 512 exceeds source size 13 [-Werror=stringop-overread]
  232 |         if (strnlen(name, KSYM_NAME_LEN) >= KSYM_NAME_LEN)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
```

Refer to the implementation of fortify's strnlen(). If the string length
is a compile-time constant, do not call the strnlen() function.

Signed-off-by: Xie Yuanbin <xieyuanbin1@huawei.com>
---
 arch/powerpc/include/asm/text-patching.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/text-patching.h b/arch/powerpc/include/asm/text-patching.h
index e7f14720f630..ce1b2131980a 100644
--- a/arch/powerpc/include/asm/text-patching.h
+++ b/arch/powerpc/include/asm/text-patching.h
@@ -228,8 +228,13 @@ static inline unsigned long ppc_kallsyms_lookup_name(const char *name)
 	/* check for dot variant */
 	char dot_name[1 + KSYM_NAME_LEN];
 	bool dot_appended = false;
+	size_t n_len = __compiletime_strlen(name);
+	const size_t n_size = __member_size(name);
 
-	if (strnlen(name, KSYM_NAME_LEN) >= KSYM_NAME_LEN)
+	if (n_len == SIZE_MAX || KSYM_NAME_LEN < n_size)
+		n_len = strnlen(name, KSYM_NAME_LEN);
+	
+	if (n_len >= KSYM_NAME_LEN)
 		return 0;
 
 	if (name[0] != '.') {
-- 
2.51.0