[v2] libbpf: workaround -Wmaybe-uninitialized false positive

Commit Message

Sam James Aug. 9, 2024, 5:26 p.m. UTC

In `elf_close`, we get this with GCC 15 -O3 (at least):
```
In function ‘elf_close’,
    inlined from ‘elf_close’ at elf.c:53:6,
    inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
elf.c:57:9: warning: ‘elf_fd.elf’ may be used uninitialized [-Wmaybe-uninitialized]
   57 |         elf_end(elf_fd->elf);
      |         ^~~~~~~~~~~~~~~~~~~~
elf.c: In function ‘elf_find_func_offset_from_file’:
elf.c:377:23: note: ‘elf_fd.elf’ was declared here
  377 |         struct elf_fd elf_fd;
      |                       ^~~~~~
In function ‘elf_close’,
    inlined from ‘elf_close’ at elf.c:53:6,
    inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
elf.c:58:9: warning: ‘elf_fd.fd’ may be used uninitialized [-Wmaybe-uninitialized]
   58 |         close(elf_fd->fd);
      |         ^~~~~~~~~~~~~~~~~
elf.c: In function ‘elf_find_func_offset_from_file’:
elf.c:377:23: note: ‘elf_fd.fd’ was declared here
  377 |         struct elf_fd elf_fd;
      |                       ^~~~~~
```

In reality, our use is fine, it's just that GCC doesn't model errno
here (see linked GCC bug). Suppress -Wmaybe-uninitialized accordingly.

Link: https://gcc.gnu.org/PR114952
Signed-off-by: Sam James <[e-mail]>
---
v2: Fix Clang build.

Range-diff against v1:
1:  3ebbe7a4e93a ! 1:  8f5c3b173e4c libbpf: workaround -Wmaybe-uninitialized false positive
    @@ tools/lib/bpf/elf.c: long elf_find_func_offset(Elf *elf, const char *binary_path
      	return ret;
      }
      
    ++#if !defined(__clang__)
     +#pragma GCC diagnostic push
     +/* https://gcc.gnu.org/PR114952 */
     +#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
    ++#endif
      /* Find offset of function name in ELF object specified by path. "name" matches
       * symbol name or name@@LIB for library functions.
       */
    @@ tools/lib/bpf/elf.c: long elf_find_func_offset_from_file(const char *binary_path
      	elf_close(&elf_fd);
      	return ret;
      }
    ++#if !defined(__clang__)
     +#pragma GCC diagnostic pop
    ++#endif
      
      struct symbol {
      	const char *name;

 tools/lib/bpf/elf.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

Alan Maguire Aug. 9, 2024, 5:48 p.m. UTC | #1

On 09/08/2024 18:26, Sam James wrote:
> In `elf_close`, we get this with GCC 15 -O3 (at least):
> ```
> In function ‘elf_close’,
>     inlined from ‘elf_close’ at elf.c:53:6,
>     inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
> elf.c:57:9: warning: ‘elf_fd.elf’ may be used uninitialized [-Wmaybe-uninitialized]
>    57 |         elf_end(elf_fd->elf);
>       |         ^~~~~~~~~~~~~~~~~~~~
> elf.c: In function ‘elf_find_func_offset_from_file’:
> elf.c:377:23: note: ‘elf_fd.elf’ was declared here
>   377 |         struct elf_fd elf_fd;
>       |                       ^~~~~~
> In function ‘elf_close’,
>     inlined from ‘elf_close’ at elf.c:53:6,
>     inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
> elf.c:58:9: warning: ‘elf_fd.fd’ may be used uninitialized [-Wmaybe-uninitialized]
>    58 |         close(elf_fd->fd);
>       |         ^~~~~~~~~~~~~~~~~
> elf.c: In function ‘elf_find_func_offset_from_file’:
> elf.c:377:23: note: ‘elf_fd.fd’ was declared here
>   377 |         struct elf_fd elf_fd;
>       |                       ^~~~~~
> ```
> 
> In reality, our use is fine, it's just that GCC doesn't model errno
> here (see linked GCC bug). Suppress -Wmaybe-uninitialized accordingly.
> 
> Link: https://gcc.gnu.org/PR114952
> Signed-off-by: Sam James <[e-mail]>
> ---
> v2: Fix Clang build.
> 
> Range-diff against v1:
> 1:  3ebbe7a4e93a ! 1:  8f5c3b173e4c libbpf: workaround -Wmaybe-uninitialized false positive
>     @@ tools/lib/bpf/elf.c: long elf_find_func_offset(Elf *elf, const char *binary_path
>       	return ret;
>       }
>       
>     ++#if !defined(__clang__)
>      +#pragma GCC diagnostic push
>      +/* https://gcc.gnu.org/PR114952 */
>      +#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
>     ++#endif
>       /* Find offset of function name in ELF object specified by path. "name" matches
>        * symbol name or name@@LIB for library functions.
>        */
>     @@ tools/lib/bpf/elf.c: long elf_find_func_offset_from_file(const char *binary_path
>       	elf_close(&elf_fd);
>       	return ret;
>       }
>     ++#if !defined(__clang__)
>      +#pragma GCC diagnostic pop
>     ++#endif
>       
>       struct symbol {
>       	const char *name;
> 
>  tools/lib/bpf/elf.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
> index c92e02394159..7058425ca85b 100644
> --- a/tools/lib/bpf/elf.c
> +++ b/tools/lib/bpf/elf.c
> @@ -369,6 +369,11 @@ long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
>  	return ret;
>  }
>  
> +#if !defined(__clang__)
> +#pragma GCC diagnostic push
> +/* https://gcc.gnu.org/PR114952 */
> +#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
> +#endif
>  /* Find offset of function name in ELF object specified by path. "name" matches
>   * symbol name or name@@LIB for library functions.
>   */
> @@ -384,6 +389,9 @@ long elf_find_func_offset_from_file(const char *binary_path, const char *name)
>  	elf_close(&elf_fd);
>  	return ret;
>  }
> +#if !defined(__clang__)
> +#pragma GCC diagnostic pop
> +#endif
>  
>  struct symbol {
>  	const char *name;


Would just initializing struct elf_fd be enough to silence the error
perhaps, i.e.

diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
index c92e02394159..3060597a527e 100644
--- a/tools/lib/bpf/elf.c
+++ b/tools/lib/bpf/elf.c
@@ -374,7 +374,7 @@ long elf_find_func_offset(Elf *elf, const char
*binary_path, const char *name)
  */
 long elf_find_func_offset_from_file(const char *binary_path, const char
*name)
 {
-       struct elf_fd elf_fd;
+       struct elf_fd elf_fd = { .fd = -1 };
        long ret = -ENOENT;

        ret = elf_open(binary_path, &elf_fd);

Sam James Aug. 12, 2024, 10:40 a.m. UTC | #2

Alan Maguire <[e-mail]> writes:

> On 09/08/2024 18:26, Sam James wrote:
>> In `elf_close`, we get this with GCC 15 -O3 (at least):
> [...]
>
> Would just initializing struct elf_fd be enough to silence the error
> perhaps, i.e.

Yeah, that WFM. Thanks, sent v3.

Patch

diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
index c92e02394159..7058425ca85b 100644
--- a/tools/lib/bpf/elf.c
+++ b/tools/lib/bpf/elf.c
@@ -369,6 +369,11 @@  long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
 	return ret;
 }
 
+#if !defined(__clang__)
+#pragma GCC diagnostic push
+/* https://gcc.gnu.org/PR114952 */
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
 /* Find offset of function name in ELF object specified by path. "name" matches
  * symbol name or name@@LIB for library functions.
  */
@@ -384,6 +389,9 @@  long elf_find_func_offset_from_file(const char *binary_path, const char *name)
 	elf_close(&elf_fd);
 	return ret;
 }
+#if !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
 
 struct symbol {
 	const char *name;