* [PATCH] exec: avoid gcc-8 warning for get_task_comm
@ 2017-12-04 14:49 Arnd Bergmann
2017-12-04 18:37 ` Kees Cook
0 siblings, 1 reply; 3+ messages in thread
From: Arnd Bergmann @ 2017-12-04 14:49 UTC (permalink / raw)
To: Alexander Viro
Cc: Arnd Bergmann, Kees Cook, Serge Hallyn, James Morris,
Ingo Molnar, Andrew Morton, Eric W. Biederman, linux-fsdevel,
linux-kernel
gcc-8 warns about using strncpy() with the source size as the limit:
fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
This is indeed slightly suspicious, as it protects us from source
arguments without NUL-termination, but does not guarantee that the
destination is terminated.
This changes it to strlcpy with a hardcoded length, to guarantee
a properly terminated string. Since we already use strlcpy() for
__set_task_comm(), the source should always be terminated properly,
so this patch won't change the behavior, but make it a bit more robust.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
fs/exec.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index 6be2aa0ab26f..3e8012afe440 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1218,9 +1218,9 @@ static int de_thread(struct task_struct *tsk)
char *get_task_comm(char *buf, struct task_struct *tsk)
{
- /* buf must be at least sizeof(tsk->comm) in size */
+ /* buf must be at least TASK_COMM_LEN in size */
task_lock(tsk);
- strncpy(buf, tsk->comm, sizeof(tsk->comm));
+ strlcpy(buf, tsk->comm, TASK_COMM_LEN);
task_unlock(tsk);
return buf;
}
--
2.9.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] exec: avoid gcc-8 warning for get_task_comm
2017-12-04 14:49 [PATCH] exec: avoid gcc-8 warning for get_task_comm Arnd Bergmann
@ 2017-12-04 18:37 ` Kees Cook
2017-12-05 13:38 ` Arnd Bergmann
0 siblings, 1 reply; 3+ messages in thread
From: Kees Cook @ 2017-12-04 18:37 UTC (permalink / raw)
To: Arnd Bergmann
Cc: Alexander Viro, Serge Hallyn, James Morris, Ingo Molnar,
Andrew Morton, Eric W. Biederman, linux-fsdevel, LKML
On Mon, Dec 4, 2017 at 6:49 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> gcc-8 warns about using strncpy() with the source size as the limit:
>
> fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
>
> This is indeed slightly suspicious, as it protects us from source
> arguments without NUL-termination, but does not guarantee that the
> destination is terminated.
>
> This changes it to strlcpy with a hardcoded length, to guarantee
> a properly terminated string. Since we already use strlcpy() for
> __set_task_comm(), the source should always be terminated properly,
It's terminated but it's not padded. This change _might_ result in
leaving a string buffer uninitialized (and if that buffer is passed as
a whole instead of just its strlen(), this could be a problem:
proc_comm_connector() e.g. may suffer from this, I haven't checked
closely).
> so this patch won't change the behavior, but make it a bit more robust.
I would rather we change get_task_comm() to include the size of buf...
maybe just convert it to a macro:
-char *get_task_comm(char *buf, struct task_struct *tsk)
+char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
...
#define get_task_comm(buf, tsk) __get_task_comm(buf, sizeof(buf), tsk)
?
That will only work for preallocated arrays though. Maybe a build bug
when sizeof(buf) != TASK_COMM_LEN ?
I think the strncpy() should stay, but for other conversions where
NULL-padding isn't an issue, strscpy() is likely preferred to
strlcpy() to avoid the potential "overread" due to the strlen() call
in strlcpy().
-Kees
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> fs/exec.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 6be2aa0ab26f..3e8012afe440 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1218,9 +1218,9 @@ static int de_thread(struct task_struct *tsk)
>
> char *get_task_comm(char *buf, struct task_struct *tsk)
> {
> - /* buf must be at least sizeof(tsk->comm) in size */
> + /* buf must be at least TASK_COMM_LEN in size */
> task_lock(tsk);
> - strncpy(buf, tsk->comm, sizeof(tsk->comm));
> + strlcpy(buf, tsk->comm, TASK_COMM_LEN);
> task_unlock(tsk);
> return buf;
> }
> --
> 2.9.0
>
--
Kees Cook
Pixel Security
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] exec: avoid gcc-8 warning for get_task_comm
2017-12-04 18:37 ` Kees Cook
@ 2017-12-05 13:38 ` Arnd Bergmann
0 siblings, 0 replies; 3+ messages in thread
From: Arnd Bergmann @ 2017-12-05 13:38 UTC (permalink / raw)
To: Kees Cook
Cc: Alexander Viro, Serge Hallyn, James Morris, Ingo Molnar,
Andrew Morton, Eric W. Biederman, linux-fsdevel, LKML
On Mon, Dec 4, 2017 at 7:37 PM, Kees Cook <keescook@chromium.org> wrote:
> On Mon, Dec 4, 2017 at 6:49 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>> gcc-8 warns about using strncpy() with the source size as the limit:
>>
>> fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
>>
>> This is indeed slightly suspicious, as it protects us from source
>> arguments without NUL-termination, but does not guarantee that the
>> destination is terminated.
>>
>> This changes it to strlcpy with a hardcoded length, to guarantee
>> a properly terminated string. Since we already use strlcpy() for
>> __set_task_comm(), the source should always be terminated properly,
>
> It's terminated but it's not padded. This change _might_ result in
> leaving a string buffer uninitialized (and if that buffer is passed as
> a whole instead of just its strlen(), this could be a problem:
> proc_comm_connector() e.g. may suffer from this, I haven't checked
> closely).
>
>> so this patch won't change the behavior, but make it a bit more robust.
>
> I would rather we change get_task_comm() to include the size of buf...
> maybe just convert it to a macro:
>
> -char *get_task_comm(char *buf, struct task_struct *tsk)
> +char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
>
> ...
>
> #define get_task_comm(buf, tsk) __get_task_comm(buf, sizeof(buf), tsk)
>
> ?
>
> That will only work for preallocated arrays though. Maybe a build bug
> when sizeof(buf) != TASK_COMM_LEN ?
Interesting idea, I'll give that a try.
> I think the strncpy() should stay, but for other conversions where
> NULL-padding isn't an issue, strscpy() is likely preferred to
> strlcpy() to avoid the potential "overread" due to the strlen() call
> in strlcpy().
Ok, I'll keep that in mind if I do more of those conversions.
gcc-8 has started warning about over 100 drivers that use
strncpy() incorrectly, and I suppose a lot of them should
actually use strscpy().
Arnd
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