linux-kernel - Re: [PATCH 0/2] scsi: fix 2 cases of -Wfortify-source

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Date:   Tue, 29 Aug 2023 09:33:55 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Tom Rix <trix@...hat.com>, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Ard Biesheuvel <ardb@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        Hannes Reinecke <hare@...nel.org>
Subject: Re: [PATCH 0/2] scsi: fix 2 cases of -Wfortify-source

On Mon, Aug 28, 2023 at 4:41 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Mon, Aug 28, 2023 at 3:25 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > clang-18 has improved its support for detecting operations that will
> > truncate values at runtime via -wfortify-source resulting in two new
>
> ^ -Wfortify-source
>
> > warnings (or errors with CONFIG_WERROR=y):
> >
> >   drivers/scsi/myrb.c:1906:10: warning: 'snprintf' will always be
> >   truncated; specified size is 32, but format string expands to at least
> >   34 [-Wfortify-source]
> >
> >   drivers/scsi/myrs.c:1089:10: warning: 'snprintf' will always be
> >   truncated; specified size is 32, but format string expands to at least
> >   34 [-Wfortify-source]
> >
> > When we have a string literal that does not contain any format flags,
> > rather than use snprintf (sometimes with a size that's too small), let's
> > use sprintf.
>
> Even better, Ard points out this could be strcpy (or one of the
> variants). Will send a v2 tomorrow.

Oh strcpy doesn't return the number of bytes copied, which is what the
users here need.

(The size of dst is also unknown).

Any thoughts on?

/* strcpy but return the length of src and requires a literal. */
#define strcpy_literal(dst, src) ({ \
  strcpy(dst, src ""); \
  __builtin_strlen(src); \
})

Uses a trick I learned from Abseil for ensuring that a parameter must
be a string literal. The C preprocessor will concatenate those
strings, or parsing will fail.

Then these drivers can do:

return strcpy_literal(buf, "physical device - not checking\n");

If that's not blood curdling, any thoughts on where best to place this
to hide it from others? Maybe just define it in both .c files?


>
> >
> > This is pattern is cleaned up throughout two files.
> >
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > ---
> > Nick Desaulniers (2):
> >       scsi: myrb: fix -Wfortify-source
> >       scsi: myrs: fix -Wfortify-source
> >
> >  drivers/scsi/myrb.c |  8 ++++----
> >  drivers/scsi/myrs.c | 14 +++++++-------
> >  2 files changed, 11 insertions(+), 11 deletions(-)
> > ---
> > base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
> > change-id: 20230828-scsi_fortify-9f8d279bf9aa
> >
> > Best regards,
> > --
> > Nick Desaulniers <ndesaulniers@...gle.com>
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers