Re: [PATCH 11/13] fs: fix unsigned enum warning with gcc-4.2

Brendan Gregg
2017-01-03T22:47:09Z

On Fri, Dec 16, 2016 at 2:56 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>
> With arm-linux-gcc-4.2, almost every file we build in the kernel ends
> up with this warning:
>
> include/linux/fs.h:2648: warning: comparison of unsigned expression < 0 is always false
>

Thanks, I'd like to see this fixed as a similar warning gets printed
whenever running many of the bcc/BPF tools, which gets annoying and is
user-visible. eg:

# /usr/share/bcc/tools/xfsslower 1
In file included from /virtual/main.c:3:
/lib/modules/4.8.6-300.fc25.x86_64/build/include/linux/fs.h:2677:9:
warning: comparison of unsigned enum expression < 0 is always false
[-Wtautological-compare]
if (id < 0 || id >= READING_MAX_ID)
~~ ^ ~
1 warning generated.
Tracing XFS operations slower than 1 ms
TIME COMM PID T BYTES OFF_KB LAT(ms) FILENAME
14:44:27 cksum 4414 R 65536 0 1.02 chcon
14:44:27 cksum 4414 R 65536 0 1.20 cpio
14:44:27 cksum 4414 R 65536 0 1.01 diff
14:44:27 cksum 4414 R 65536 0 1.15 dir
[...]

This patch fixes the warning.

Brendan

>
> Later versions don't have this problem, but it's easy enough to
> work around.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> include/linux/fs.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 398cf20a706d..782c2a292fd7 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2645,7 +2645,7 @@ static const char * const kernel_read_file_str[] = {
>
> static inline const char *kernel_read_file_id_str(enum kernel_read_file_id id)
> {
> - if (id < 0 || id >= READING_MAX_ID)
> + if ((unsigned)id >= READING_MAX_ID)
> return kernel_read_file_str[READING_UNKNOWN];
>
> return kernel_read_file_str[id];
> --
> 2.9.0
>