Re: clang: net: qed_main.c:1227:3: error: 'snprintf' will always be truncated; specified size is 16, but format string expands to at least 18 [-Werror,-Wfortify-source]

[Nathan Chancellor <nathan@kernel.org>]

Hi Naresh,

On Mon, Aug 28, 2023 at 05:57:38PM +0530, Naresh Kamboju wrote:
> [My two cents]
> 
> stable-rc linux-6.1.y and linux-6.4.y x86 clang-nightly builds fail with
> following warnings / errors.
> 
> Build errors:
> --------------
> drivers/net/ethernet/qlogic/qed/qed_main.c:1227:3: error: 'snprintf'
> will always be truncated; specified size is 16, but format string
> expands to at least 18 [-Werror,-Wfortify-source]
>  1227 |                 snprintf(name, NAME_SIZE, "slowpath-%02x:%02x.%02x",
>       |                 ^
> 1 error generated.
> 
> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>

Thank you as always for the report. This is a result of a change in
clang to implement the equivalent of GCC's -Wformat-truncation, which is
currently disabled for the whole kernel in mainline and enabled in W=1
in -next. I have filed an issue to figure out what to do about this:
https://github.com/ClangBuiltLinux/linux/issues/1923

For the record, if you see an issue with clang-nightly that you do not
see with older versions of clang, it is generally an indication that
something has changed on the toolchain side, so it is probably not worth
bothering the stable or subsystem folks with the initial report.
Consider just messaging Nick, myself, and llvm@lists.linux.dev in those
cases so we can pre-triage and bring other folks in as necessary.

That said, this seems like a legitimate warning. As I mentioned above,
GCC shows the same warning with W=1 in -next, so this should be fixed.

  drivers/net/ethernet/qlogic/qed/qed_main.c: In function 'qed_slowpath_start':
  drivers/net/ethernet/qlogic/qed/qed_main.c:1218:63: error: '%02x' directive output truncated writing 2 bytes into a region of size 1 [-Werror=format-truncation=]
   1218 |                 snprintf(name, NAME_SIZE, "slowpath-%02x:%02x.%02x",
        |                                                               ^~~~
  In function 'qed_slowpath_wq_start',
      inlined from 'qed_slowpath_start' at drivers/net/ethernet/qlogic/qed/qed_main.c:1250:6:
  drivers/net/ethernet/qlogic/qed/qed_main.c:1218:43: note: directive argument in the range [0, 255]
   1218 |                 snprintf(name, NAME_SIZE, "slowpath-%02x:%02x.%02x",
        |                                           ^~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/net/ethernet/qlogic/qed/qed_main.c:1218:17: note: 'snprintf' output 18 bytes into a destination of size 16
   1218 |                 snprintf(name, NAME_SIZE, "slowpath-%02x:%02x.%02x",
        |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   1219 |                          cdev->pdev->bus->number,
        |                          ~~~~~~~~~~~~~~~~~~~~~~~~
   1220 |                          PCI_SLOT(cdev->pdev->devfn), hwfn->abs_pf_id);
        |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Cheers,
Nathan