Re: [PATCH v2] tools/9pfsd: Fix build error caused by strerror_r()

[Juergen Gross <jgross@xxxxxxxx>]

On 07.03.24 11:38, Henry Wang wrote:
> Below error can be seen when doing Yocto build of the toolstack:
>
> | io.c: In function 'p9_error':
> | io.c:684:5: error: ignoring return value of 'strerror_r' declared
>    with attribute 'warn_unused_result' [-Werror=unused-result]
> |   684 |     strerror_r(err, ring->buffer, ring->ring_size);
> |       |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> | cc1: all warnings being treated as errors
>
> Fix the build by using strerror() to replace strerror_r(). Since
> strerror() is thread-unsafe, use a separate local mutex to protect
> the action. The steps would then become: Acquire the mutex first,
> invoke strerror(), copy the string from strerror() to the designated
> buffer and then drop the mutex.
>
> Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
Maybe add a "Fixes:" tag referencing Jan's patch?

And I would expand on the reason why you are using strerror() instead of just
checking the strerror_r() result. Something like:

  Using strerror_r() without special casing different build
  environments is impossible due to the different return types
  (int vs char *) depending on the environment. As p9_error()
  is not on a performance critical path, using strerror() with a
  mutex ought to be fine.

> ---
>   tools/9pfsd/io.c | 12 +++++++++++-
>   1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/tools/9pfsd/io.c b/tools/9pfsd/io.c
> index adb887c7d9..2b80c9528d 100644
> --- a/tools/9pfsd/io.c
> +++ b/tools/9pfsd/io.c
> @@ -680,8 +680,18 @@ static bool name_ok(const char *str)
>   static void p9_error(struct ring *ring, uint16_t tag, uint32_t err)
>   {
>       unsigned int erroff;
> +    static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
> +    char *strerror_str;
> +    RING_IDX strerror_len = 0, copy_len = 0;
I wouldn't use RING_IDX for the type, but unsigned int.

> +
> +    pthread_mutex_lock(&mutex);
> +    strerror_str = strerror(err);
> +    strerror_len = strlen(strerror_str) + 1;
> +    copy_len = min(strerror_len, ring->ring_size);
> +    memcpy(ring->buffer, strerror_str, copy_len);
> +    ((char *)(ring->buffer))[copy_len - 1] = '\0';
> +    pthread_mutex_unlock(&mutex);
>   
> -    strerror_r(err, ring->buffer, ring->ring_size);
>       erroff = add_string(ring, ring->buffer, strlen(ring->buffer));
>       fill_buffer(ring, P9_CMD_ERROR, tag, "SU",
>                   erroff != ~0 ? ring->str + erroff : "cannot allocate memory",

Juergen

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