linux-kernel - Re: [PATCH] can: mcp251xfd: silence clang's -Wunaligned-access warning

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Date:   Wed, 18 May 2022 09:18:19 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>,
        kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
        kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org,
        linux-can@...r.kernel.org
Subject: Re: [PATCH] can: mcp251xfd: silence clang's -Wunaligned-access
 warning

On Thu, May 19, 2022 at 01:15:04AM +0900, Vincent MAILHOL wrote:
> On Tue. 19 May 2022 at 01:08, Nathan Chancellor <nathan@...nel.org> wrote:
> > Hi Vincent,
> >
> > On Wed, May 18, 2022 at 08:43:57PM +0900, Vincent Mailhol wrote:
> > > clang emits a -Wunaligned-access warning on union
> > > mcp251xfd_tx_ojb_load_buf.
> > >
> > > The reason is that field hw_tx_obj (not declared as packed) is being
> > > packed right after a 16 bits field inside a packed struct:
> > >
> > > | union mcp251xfd_tx_obj_load_buf {
> > > |     struct __packed {
> > > |             struct mcp251xfd_buf_cmd cmd;
> > > |               /* ^ 16 bits fields */
> > > |             struct mcp251xfd_hw_tx_obj_raw hw_tx_obj;
> > > |               /* ^ not declared as packed */
> > > |     } nocrc;
> > > |     struct __packed {
> > > |             struct mcp251xfd_buf_cmd_crc cmd;
> > > |             struct mcp251xfd_hw_tx_obj_raw hw_tx_obj;
> > > |             __be16 crc;
> > > |     } crc;
> > > | } ____cacheline_aligned;
> > >
> > > Starting from LLVM 14, having an unpacked struct nested in a packed
> > > struct triggers a warning. c.f. [1].
> > >
> > > This is a false positive because the field is always being accessed
> > > with the relevant put_unaligned_*() function. Adding __packed to the
> > > structure declaration silences the warning.
> > >
> > > [1] https://github.com/llvm/llvm-project/issues/55520
> > >
> > > Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> > > ---
> > > Actually, I do not have llvm 14 installed so I am not able to test
> > > (this check was introduced in v14). But as explained in [1], adding
> > > __packed should fix the warning.
> >
> > Thanks for the patch! This does resolve the warning (verified with LLVM
> > 15).
> 
> Great, thanks for the check! Does this mean we can add you Tested-by
> (I assume yes, c.f. below, if not the case, please raise your voice).

Sure, see below.

> > > Because this is a false positive, I did not add a Fixes tag, nor a
> > > Reported-by: kernel test robot.
> >
> > I think that the Reported-by tag should always be included but I agree
> > that a Fixes tag is not necessary for this warning, as we currently have
> > it under W=1, so it should not be visible under normal circumstances.
> 
> ACK.
> Marc, can you directly add below tags to the patch:
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Tested-by: Nathan Chancellor <nathan@...nel.org>

Please use:

Tested-by: Nathan Chancellor <nathan@...nel.org> # build

To make it clear that I didn't perform anything more than a build test
to see that the warning is fixed.

Cheers,
Nathan