[PATCH] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[PATCH] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Mikhail Gavrilov]

glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
-Wdiscarded-qualifiers to an error in the default hardening flags
of Fedora Rawhide, Arch Linux, openSUSE Tumbleweed, Gentoo, etc.

In C23, strstr() and strchr() return "const char *" in most cases,
making implicit casts from const to non-const invalid.

This breaks the build of tools/bpf/resolve_btfids on pristine
upstream kernel when using GCC 15 + glibc 2.42+.

Fix the three remaining instances with explicit casts.

No functional changes.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2417601
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---
 tools/lib/bpf/libbpf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index dd3b2f57082d..dd11feef3adf 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8247,7 +8247,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
 	struct extern_desc *ext;
 	char *res;
 
-	res = strstr(sym_name, ".llvm.");
+	res = (char *)strstr(sym_name, ".llvm.");
 	if (sym_type == 'd' && res)
 		ext = find_extern_by_name_with_len(obj, sym_name, res - sym_name);
 	else
@@ -11576,7 +11576,7 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
 		 */
 		char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;
 
-		if (!(sym_sfx = strstr(sym_name, ".llvm.")))
+		if (!(sym_sfx = (char *)strstr(sym_name, ".llvm.")))
 			return 0;
 
 		/* psym_trim vs sym_trim dance is done to avoid pointer vs array
@@ -12164,7 +12164,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 
 			if (s[0] == ':')
 				s++;
-			next_path = strchr(s, ':');
+			next_path = (char *)strchr(s, ':');
 			seg_len = next_path ? next_path - s : strlen(s);
 			if (!seg_len)
 				continue;
-- 
2.52.0

Re: [PATCH] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Florian Weimer]

* Mikhail Gavrilov:

> glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
> -Wdiscarded-qualifiers to an error in the default hardening flags
> of Fedora Rawhide, Arch Linux, openSUSE Tumbleweed, Gentoo, etc.
>
> In C23, strstr() and strchr() return "const char *" in most cases,
> making implicit casts from const to non-const invalid.
>
> This breaks the build of tools/bpf/resolve_btfids on pristine
> upstream kernel when using GCC 15 + glibc 2.42+.
>
> Fix the three remaining instances with explicit casts.
>
> No functional changes.
>
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2417601
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
> ---
>  tools/lib/bpf/libbpf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index dd3b2f57082d..dd11feef3adf 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8247,7 +8247,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
>  	struct extern_desc *ext;
>  	char *res;
>  
> -	res = strstr(sym_name, ".llvm.");
> +	res = (char *)strstr(sym_name, ".llvm.");
>  	if (sym_type == 'd' && res)
>  		ext = find_extern_by_name_with_len(obj, sym_name, res - sym_name);
>  	else
> @@ -11576,7 +11576,7 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
>  		 */
>  		char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;
>  
> -		if (!(sym_sfx = strstr(sym_name, ".llvm.")))
> +		if (!(sym_sfx = (char *)strstr(sym_name, ".llvm.")))
>  			return 0;
>  
>  		/* psym_trim vs sym_trim dance is done to avoid pointer vs array
> @@ -12164,7 +12164,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
>  
>  			if (s[0] == ':')
>  				s++;
> -			next_path = strchr(s, ':');
> +			next_path = (char *)strchr(s, ':');
>  			seg_len = next_path ? next_path - s : strlen(s);
>  			if (!seg_len)
>  				continue;

I think you should change the type of the relevant variables to const
char *.  The kernel coding style does not disallow using const, does it?

Thanks,
Florian

[PATCH v2 bpf-next] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Mikhail Gavrilov]

glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
-Wdiscarded-qualifiers to an error in the default hardening flags
of Fedora Rawhide, Arch Linux, openSUSE Tumbleweed, Gentoo, etc.

In C23, strstr() and strchr() return "const char *" in most cases,
making previous implicit casts invalid.

This breaks the build of tools/bpf/resolve_btfids on pristine
upstream kernel when using GCC 15 + glibc 2.42+.

Fix the three remaining instances with explicit casts.

No functional changes.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2417601
Suggested-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

---
v2:
- Use explicit casts instead of changing variable types to const char *,
  because the variables are already declared as char * earlier in the
  functions and used in contexts requiring mutability.
  This is common practice in the kernel when full const-correctness
  cannot be preserved without major refactoring.
---
 tools/lib/bpf/libbpf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index dd3b2f57082d..dd11feef3adf 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8247,7 +8247,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
 	struct extern_desc *ext;
 	char *res;
 
-	res = strstr(sym_name, ".llvm.");
+	res = (char *)strstr(sym_name, ".llvm.");
 	if (sym_type == 'd' && res)
 		ext = find_extern_by_name_with_len(obj, sym_name, res - sym_name);
 	else
@@ -11576,7 +11576,7 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
 		 */
 		char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;
 
-		if (!(sym_sfx = strstr(sym_name, ".llvm.")))
+		if (!(sym_sfx = (char *)strstr(sym_name, ".llvm.")))
 			return 0;
 
 		/* psym_trim vs sym_trim dance is done to avoid pointer vs array
@@ -12164,7 +12164,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 
 			if (s[0] == ':')
 				s++;
-			next_path = strchr(s, ':');
+			next_path = (char *)strchr(s, ':');
 			seg_len = next_path ? next_path - s : strlen(s);
 			if (!seg_len)
 				continue;
-- 
2.52.0

Re: [PATCH v2 bpf-next] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Florian Weimer]

* Mikhail Gavrilov:

> - Use explicit casts instead of changing variable types to const char *,
>   because the variables are already declared as char * earlier in the
>   functions and used in contexts requiring mutability.
>   This is common practice in the kernel when full const-correctness
>   cannot be preserved without major refactoring.

What kind of mutability is this about?  Obviously char *const won't
work, but const char * seems to be fine for these variables?

Thanks,
Florian

[PATCH v2 bpf-next] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Mikhail Gavrilov]

glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
-Wdiscarded-qualifiers to an error in the default hardening flags
of Fedora Rawhide, Arch Linux, openSUSE Tumbleweed, Gentoo, etc.

In C23, strstr() and strchr() return "const char *" in most cases,
making previous implicit casts invalid.

This breaks the build of tools/bpf/resolve_btfids on pristine
upstream kernel when using GCC 15 + glibc 2.42+.

Fix the three remaining instances with explicit casts.

No functional changes.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2417601
Suggested-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

---
v2:
- Declare `res` as `const char *` — never modified.
- Keep `sym_sfx` as `char *` and cast — it is advanced in the loop.
- Cast `next_path` — declared as `char *` earlier in the function.
  Changing it to const would require refactoring the whole function,
  which is not justified for a tools/ file.
---
 tools/lib/bpf/libbpf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index dd3b2f57082d..22ccd50e9978 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8245,7 +8245,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
 	struct bpf_object *obj = ctx;
 	const struct btf_type *t;
 	struct extern_desc *ext;
-	char *res;
+	const char *res;
 
 	res = strstr(sym_name, ".llvm.");
 	if (sym_type == 'd' && res)
@@ -11576,7 +11576,7 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
 		 */
 		char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;
 
-		if (!(sym_sfx = strstr(sym_name, ".llvm.")))
+		if (!(sym_sfx = (char *)strstr(sym_name, ".llvm.")))  /* needs mutation */
 			return 0;
 
 		/* psym_trim vs sym_trim dance is done to avoid pointer vs array
@@ -12164,7 +12164,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 
 			if (s[0] == ':')
 				s++;
-			next_path = strchr(s, ':');
+			next_path = (char *)strchr(s, ':');   /* declared as char * above */
 			seg_len = next_path ? next_path - s : strlen(s);
 			if (!seg_len)
 				continue;
-- 
2.52.0

Re: [PATCH v2 bpf-next] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Andrii Nakryiko]

On Fri, Nov 28, 2025 at 6:59 AM Mikhail Gavrilov
<mikhail.v.gavrilov@gmail.com> wrote:
>
> glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
> -Wdiscarded-qualifiers to an error in the default hardening flags
> of Fedora Rawhide, Arch Linux, openSUSE Tumbleweed, Gentoo, etc.
>
> In C23, strstr() and strchr() return "const char *" in most cases,
> making previous implicit casts invalid.
>
> This breaks the build of tools/bpf/resolve_btfids on pristine
> upstream kernel when using GCC 15 + glibc 2.42+.
>
> Fix the three remaining instances with explicit casts.
>
> No functional changes.
>
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2417601
> Suggested-by: Florian Weimer <fweimer@redhat.com>
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
>
> ---
> v2:
> - Declare `res` as `const char *` — never modified.
> - Keep `sym_sfx` as `char *` and cast — it is advanced in the loop.
> - Cast `next_path` — declared as `char *` earlier in the function.
>   Changing it to const would require refactoring the whole function,
>   which is not justified for a tools/ file.
> ---
>  tools/lib/bpf/libbpf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index dd3b2f57082d..22ccd50e9978 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8245,7 +8245,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
>         struct bpf_object *obj = ctx;
>         const struct btf_type *t;
>         struct extern_desc *ext;
> -       char *res;
> +       const char *res;
>
>         res = strstr(sym_name, ".llvm.");
>         if (sym_type == 'd' && res)
> @@ -11576,7 +11576,7 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
>                  */
>                 char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;

const char *sym_sfx; instead of unnecessary cast

>
> -               if (!(sym_sfx = strstr(sym_name, ".llvm.")))
> +               if (!(sym_sfx = (char *)strstr(sym_name, ".llvm.")))  /* needs mutation */
>                         return 0;
>
>                 /* psym_trim vs sym_trim dance is done to avoid pointer vs array
> @@ -12164,7 +12164,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
>
>                         if (s[0] == ':')
>                                 s++;
> -                       next_path = strchr(s, ':');
> +                       next_path = (char *)strchr(s, ':');   /* declared as char * above */

same here, next_path should be const char *

pw-bot: cr


>                         seg_len = next_path ? next_path - s : strlen(s);
>                         if (!seg_len)
>                                 continue;
> --
> 2.52.0
>

[PATCH v3] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Mikhail Gavrilov]

glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
-Wdiscarded-qualifiers to an error.

In C23, strstr() and strchr() return "const char *".

Declare `res` and `next_path` as const char * — they are never modified.
Keep `sym_sfx` as char * because it is advanced in a loop.

Suggested-by: Florian Weimer <fweimer@redhat.com>
Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---
v2: use const char * where possible (Florian, Andrii)
v3: declare res/next_path as const char * (never modified)
    keep cast for sym_sfx (advanced in loop)
---
 tools/lib/bpf/libbpf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 3dc8a8078815..81782471f1d0 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8484,7 +8484,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
 	struct bpf_object *obj = ctx;
 	const struct btf_type *t;
 	struct extern_desc *ext;
-	char *res;
+	const char *res;
 
 	res = strstr(sym_name, ".llvm.");
 	if (sym_type == 'd' && res)
@@ -11820,7 +11820,7 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
 		 */
 		char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;
 
-		if (!(sym_sfx = strstr(sym_name, ".llvm.")))
+		if (!(sym_sfx = (char *)strstr(sym_name, ".llvm.")))  /* needs mutation */
 			return 0;
 
 		/* psym_trim vs sym_trim dance is done to avoid pointer vs array
@@ -12401,7 +12401,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 		if (!search_paths[i])
 			continue;
 		for (s = search_paths[i]; s != NULL; s = strchr(s, ':')) {
-			char *next_path;
+			const char *next_path;
 			int seg_len;
 
 			if (s[0] == ':')
-- 
2.52.0

[PATCH v3] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[Mikhail Gavrilov]

glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
-Wdiscarded-qualifiers to an error.

In C23, strstr() and strchr() return "const char *".

Change variable types to const char * where the pointers are never
modified (res, sym_sfx, next_path).

Suggested-by: Florian Weimer <fweimer@redhat.com>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---
v2: use const char * where possible
v3: split declaration of sym_sfx — it is only read, never written
---
 tools/lib/bpf/libbpf.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 3dc8a8078815..f4dfd23148a5 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8484,7 +8484,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
 	struct bpf_object *obj = ctx;
 	const struct btf_type *t;
 	struct extern_desc *ext;
-	char *res;
+	const char *res;
 
 	res = strstr(sym_name, ".llvm.");
 	if (sym_type == 'd' && res)
@@ -11818,7 +11818,8 @@ static int avail_kallsyms_cb(unsigned long long sym_addr, char sym_type,
 		 *
 		 *   [0] fb6a421fb615 ("kallsyms: Match symbols exactly with CONFIG_LTO_CLANG")
 		 */
-		char sym_trim[256], *psym_trim = sym_trim, *sym_sfx;
+		char sym_trim[256], *psym_trim = sym_trim;
+		const char *sym_sfx;
 
 		if (!(sym_sfx = strstr(sym_name, ".llvm.")))
 			return 0;
@@ -12401,7 +12402,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 		if (!search_paths[i])
 			continue;
 		for (s = search_paths[i]; s != NULL; s = strchr(s, ':')) {
-			char *next_path;
+			const char *next_path;
 			int seg_len;
 
 			if (s[0] == ':')
-- 
2.52.0

Re: [PATCH v3] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Sat,  6 Dec 2025 14:28:25 +0500 you wrote:
> glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
> -Wdiscarded-qualifiers to an error.
> 
> In C23, strstr() and strchr() return "const char *".
> 
> Change variable types to const char * where the pointers are never
> modified (res, sym_sfx, next_path).
> 
> [...]

Here is the summary with links:
  - [v3] tools/lib/bpf: fix -Wdiscarded-qualifiers under C23
    https://git.kernel.org/bpf/bpf/c/d70f79fef658

You are awesome, thank you!
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