[v3] libbpf: workaround -Wmaybe-uninitialized false positive

Commit Message

Sam James Aug. 12, 2024, 10:37 a.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
by initializing elf_fd.elf to -1.

I've done this in two other functions as well given it could easily
occur there too (same access/use pattern).

Link: https://gcc.gnu.org/PR114952
Signed-off-by: Sam James <[e-mail]>
---
v3: Initialize to -1 instead of using a pragma.

Range-diff against v2:
1:  8f5c3b173e4cb < -:  ------------- libbpf: workaround -Wmaybe-uninitialized false positive
-:  ------------- > 1:  12cec1262be71 libbpf: workaround -Wmaybe-uninitialized false positive

 tools/lib/bpf/elf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Comments

Jiri Olsa Aug. 12, 2024, 11:27 a.m. UTC | #1

On Mon, Aug 12, 2024 at 11:37:59AM +0100, 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
> by initializing elf_fd.elf to -1.
> 
> I've done this in two other functions as well given it could easily
> occur there too (same access/use pattern).
> 
> Link: https://gcc.gnu.org/PR114952
> Signed-off-by: Sam James <[e-mail]>
> ---
> v3: Initialize to -1 instead of using a pragma.

it's false positive, but I wonder we could still add Fixes tag

Acked-by: Jiri Olsa <[e-mail]>

jirka

> 
> Range-diff against v2:
> 1:  8f5c3b173e4cb < -:  ------------- libbpf: workaround -Wmaybe-uninitialized false positive
> -:  ------------- > 1:  12cec1262be71 libbpf: workaround -Wmaybe-uninitialized false positive
> 
>  tools/lib/bpf/elf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
> index c92e02394159e..00ea3f867bbc8 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);
> @@ -412,7 +412,7 @@ int elf_resolve_syms_offsets(const char *binary_path, int cnt,
>  	int err = 0, i, cnt_done = 0;
>  	unsigned long *offsets;
>  	struct symbol *symbols;
> -	struct elf_fd elf_fd;
> +	struct elf_fd elf_fd = { .fd = -1 };
>  
>  	err = elf_open(binary_path, &elf_fd);
>  	if (err)
> @@ -507,7 +507,7 @@ int elf_resolve_pattern_offsets(const char *binary_path, const char *pattern,
>  	int sh_types[2] = { SHT_SYMTAB, SHT_DYNSYM };
>  	unsigned long *offsets = NULL;
>  	size_t cap = 0, cnt = 0;
> -	struct elf_fd elf_fd;
> +	struct elf_fd elf_fd = { .fd = -1 };
>  	int err = 0, i;
>  
>  	err = elf_open(binary_path, &elf_fd);
> -- 
> 2.45.2
>

Alan Maguire Aug. 12, 2024, 1:56 p.m. UTC | #2

On 12/08/2024 11:37, 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
> by initializing elf_fd.elf to -1.
> 
> I've done this in two other functions as well given it could easily
> occur there too (same access/use pattern).
>

hmm, looking at this again - given that there are multiple consumers -
I suppose another option would perhaps be to

- have elf_open() to init int fd = -1, Elf *elf = NULL.
- have error paths in elf_open() "goto out"; at out: we set elf_fd->fd,
elf_fd->elf to fd, elf
- have elf_close() exit it elf_fd < 0 (since 0 is a valid fd), as it
will for the error cases

Might all be bit excessive, and might not even fix the false positive
issue here, so

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

Reviewed-by: Alan Maguire <[e-mail]>

> ---
> v3: Initialize to -1 instead of using a pragma.
> 
> Range-diff against v2:
> 1:  8f5c3b173e4cb < -:  ------------- libbpf: workaround -Wmaybe-uninitialized false positive
> -:  ------------- > 1:  12cec1262be71 libbpf: workaround -Wmaybe-uninitialized false positive
> 
>  tools/lib/bpf/elf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
> index c92e02394159e..00ea3f867bbc8 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);
> @@ -412,7 +412,7 @@ int elf_resolve_syms_offsets(const char *binary_path, int cnt,
>  	int err = 0, i, cnt_done = 0;
>  	unsigned long *offsets;
>  	struct symbol *symbols;
> -	struct elf_fd elf_fd;
> +	struct elf_fd elf_fd = { .fd = -1 };
>  
>  	err = elf_open(binary_path, &elf_fd);
>  	if (err)
> @@ -507,7 +507,7 @@ int elf_resolve_pattern_offsets(const char *binary_path, const char *pattern,
>  	int sh_types[2] = { SHT_SYMTAB, SHT_DYNSYM };
>  	unsigned long *offsets = NULL;
>  	size_t cap = 0, cnt = 0;
> -	struct elf_fd elf_fd;
> +	struct elf_fd elf_fd = { .fd = -1 };
>  	int err = 0, i;
>  
>  	err = elf_open(binary_path, &elf_fd);

Andrii Nakryiko Aug. 12, 2024, 9:02 p.m. UTC | #3

On Mon, Aug 12, 2024 at 6:57 AM Alan Maguire <[e-mail]> wrote:
>
> On 12/08/2024 11:37, 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
> > by initializing elf_fd.elf to -1.
> >
> > I've done this in two other functions as well given it could easily
> > occur there too (same access/use pattern).
> >
>
> hmm, looking at this again - given that there are multiple consumers -

yes, I don't like that each caller has to remember to initialize the
struct that is clearly initialized by elf_open() itself, so see below.

pw-bot: cr

> I suppose another option would perhaps be to
>
> - have elf_open() to init int fd = -1, Elf *elf = NULL.

I'd do just

elf_fd->elf = NULL;
elf_fd->fd = -1;

and do nothing else. This should be enough for compiler to not trigger this.

> - have error paths in elf_open() "goto out"; at out: we set elf_fd->fd,
> elf_fd->elf to fd, elf
> - have elf_close() exit it elf_fd < 0 (since 0 is a valid fd), as it
> will for the error cases
>

Let's not touch anything else, this should be enough.


> Might all be bit excessive, and might not even fix the false positive
> issue here, so
>
> > Link: https://gcc.gnu.org/PR114952
> > Signed-off-by: Sam James <[e-mail]>
>
> Reviewed-by: Alan Maguire <[e-mail]>
>
> > ---
> > v3: Initialize to -1 instead of using a pragma.
> >
> > Range-diff against v2:
> > 1:  8f5c3b173e4cb < -:  ------------- libbpf: workaround -Wmaybe-uninitialized false positive
> > -:  ------------- > 1:  12cec1262be71 libbpf: workaround -Wmaybe-uninitialized false positive
> >
> >  tools/lib/bpf/elf.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
> > index c92e02394159e..00ea3f867bbc8 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);
> > @@ -412,7 +412,7 @@ int elf_resolve_syms_offsets(const char *binary_path, int cnt,
> >       int err = 0, i, cnt_done = 0;
> >       unsigned long *offsets;
> >       struct symbol *symbols;
> > -     struct elf_fd elf_fd;
> > +     struct elf_fd elf_fd = { .fd = -1 };
> >
> >       err = elf_open(binary_path, &elf_fd);
> >       if (err)
> > @@ -507,7 +507,7 @@ int elf_resolve_pattern_offsets(const char *binary_path, const char *pattern,
> >       int sh_types[2] = { SHT_SYMTAB, SHT_DYNSYM };
> >       unsigned long *offsets = NULL;
> >       size_t cap = 0, cnt = 0;
> > -     struct elf_fd elf_fd;
> > +     struct elf_fd elf_fd = { .fd = -1 };
> >       int err = 0, i;
> >
> >       err = elf_open(binary_path, &elf_fd);

Sam James Aug. 13, 2024, 7:38 p.m. UTC | #4

Andrii Nakryiko <[e-mail]> writes:

> On Mon, Aug 12, 2024 at 6:57 AM Alan Maguire <[e-mail]> wrote:
>>
>> On 12/08/2024 11:37, 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
>> > by initializing elf_fd.elf to -1.
>> >
>> > I've done this in two other functions as well given it could easily
>> > occur there too (same access/use pattern).
>> >
>>
>> hmm, looking at this again - given that there are multiple consumers -
>
> yes, I don't like that each caller has to remember to initialize the
> struct that is clearly initialized by elf_open() itself, so see below.
>
> pw-bot: cr
>
>> I suppose another option would perhaps be to
>>
>> - have elf_open() to init int fd = -1, Elf *elf = NULL.
>
> I'd do just
>
> elf_fd->elf = NULL;
> elf_fd->fd = -1;
>
> and do nothing else. This should be enough for compiler to not trigger this.

OK.

>
>> - have error paths in elf_open() "goto out"; at out: we set elf_fd->fd,
>> elf_fd->elf to fd, elf
>> - have elf_close() exit it elf_fd < 0 (since 0 is a valid fd), as it
>> will for the error cases
>>
>
> Let's not touch anything else, this should be enough.
>
>
>> Might all be bit excessive, and might not even fix the false positive
>> issue here, so
>>
>> > Link: https://gcc.gnu.org/PR114952
>> > Signed-off-by: Sam James <[e-mail]>
>>
>> Reviewed-by: Alan Maguire <[e-mail]>
>>
>> > ---
>> > v3: Initialize to -1 instead of using a pragma.
>> >
>> > Range-diff against v2:
>> > 1:  8f5c3b173e4cb < -:  ------------- libbpf: workaround -Wmaybe-uninitialized false positive
>> > -:  ------------- > 1:  12cec1262be71 libbpf: workaround -Wmaybe-uninitialized false positive
>> >
>> >  tools/lib/bpf/elf.c | 6 +++---
>> >  1 file changed, 3 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
>> > index c92e02394159e..00ea3f867bbc8 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);
>> > @@ -412,7 +412,7 @@ int elf_resolve_syms_offsets(const char *binary_path, int cnt,
>> >       int err = 0, i, cnt_done = 0;
>> >       unsigned long *offsets;
>> >       struct symbol *symbols;
>> > -     struct elf_fd elf_fd;
>> > +     struct elf_fd elf_fd = { .fd = -1 };
>> >
>> >       err = elf_open(binary_path, &elf_fd);
>> >       if (err)
>> > @@ -507,7 +507,7 @@ int elf_resolve_pattern_offsets(const char *binary_path, const char *pattern,
>> >       int sh_types[2] = { SHT_SYMTAB, SHT_DYNSYM };
>> >       unsigned long *offsets = NULL;
>> >       size_t cap = 0, cnt = 0;
>> > -     struct elf_fd elf_fd;
>> > +     struct elf_fd elf_fd = { .fd = -1 };
>> >       int err = 0, i;
>> >
>> >       err = elf_open(binary_path, &elf_fd);

Patch

diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
index c92e02394159e..00ea3f867bbc8 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);
@@ -412,7 +412,7 @@  int elf_resolve_syms_offsets(const char *binary_path, int cnt,
 	int err = 0, i, cnt_done = 0;
 	unsigned long *offsets;
 	struct symbol *symbols;
-	struct elf_fd elf_fd;
+	struct elf_fd elf_fd = { .fd = -1 };
 
 	err = elf_open(binary_path, &elf_fd);
 	if (err)
@@ -507,7 +507,7 @@  int elf_resolve_pattern_offsets(const char *binary_path, const char *pattern,
 	int sh_types[2] = { SHT_SYMTAB, SHT_DYNSYM };
 	unsigned long *offsets = NULL;
 	size_t cap = 0, cnt = 0;
-	struct elf_fd elf_fd;
+	struct elf_fd elf_fd = { .fd = -1 };
 	int err = 0, i;
 
 	err = elf_open(binary_path, &elf_fd);