Date:   Thu, 13 May 2021 15:36:58 -0700
From:   Daniel Latypov <dlatypov@...gle.com>
To:     David Gow <davidgow@...gle.com>
Cc:     Brendan Higgins <brendanhiggins@...gle.com>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        KUnit Development <kunit-dev@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kunit: Add gnu_printf specifiers

On Thu, May 13, 2021 at 1:48 PM David Gow <davidgow@...gle.com> wrote:
>
> On Fri, May 14, 2021 at 4:25 AM Daniel Latypov <dlatypov@...gle.com> wrote:
> >
> > On Thu, May 13, 2021 at 1:03 PM 'David Gow' via KUnit Development
> > <kunit-dev@...glegroups.com> wrote:
> > >
> > > Some KUnit functions use variable arguments to implement a printf-like
> > > format string. Use the __printf() attribute to let the compiler warn if
> > > invalid format strings are passed in.
> > >
> > > If the kernel is build with W=1, it complained about the lack of these
> > > specifiers, e.g.:
> > > ../lib/kunit/test.c:72:2: warning: function ‘kunit_log_append’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
> > >
> > > Signed-off-by: David Gow <davidgow@...gle.com>
> >
> > Reviewed-by: Daniel Latypov <dlatypov@...gle.com>
> >
> > As noted below, these additions don't really do anything.
> > Unfortunately, they just make compiler warnings noisier in the case of
> > kunit_log_append().
> >
> > But if this silences a W=1 warning, then we should probably add them in.
> > I guess it also serves as documentation that we're using the same
> > standard format specifiers and not something custom, which is nice.
> >
>
> Yeah: I did this to get rid of the W=1 warnings. I don't know if
> there's a way of doing this which would be less verbose: I do think
> that the format checking is worthwhile in general, even if we're
> hitting a few nasty cases where things are nested in macros.

Yeah. In case it wasn't clear, I think both annotations are clearly
worth having if they silence W=1 warnings.
We're more likely to produce more noise w/ those warnings than the
extra noise of someone making a typo or forgetting in a kunit_info()
somewhere while writing a test.

It's just a bit sad that doing what the compiler suggests doesn't
really make life better :(

>
>
> > > ---
> > >  include/kunit/test.h      | 2 +-
> > >  lib/kunit/string-stream.h | 6 +++---
> > >  2 files changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/include/kunit/test.h b/include/kunit/test.h
> > > index 49601c4b98b8..af2e386b867c 100644
> > > --- a/include/kunit/test.h
> > > +++ b/include/kunit/test.h
> > > @@ -610,7 +610,7 @@ static inline void *kunit_kzalloc(struct kunit *test, size_t size, gfp_t gfp)
> > >
> > >  void kunit_cleanup(struct kunit *test);
> > >
> > > -void kunit_log_append(char *log, const char *fmt, ...);
> > > +void __printf(2, 3) kunit_log_append(char *log, const char *fmt, ...);
> >
> > Before this patch:
> > ../lib/kunit/kunit-example-test.c: In function ‘example_simple_test’:
> > ../include/linux/kern_levels.h:5:18: warning: format ‘%s’ expects
> > argument of type ‘char *’, but argument 3 has type ‘int’ [-Wformat=]
> >     5 | #define KERN_SOH "\001"  /* ASCII Start Of Header */
> >       |                  ^~~~~~
> > ../include/kunit/test.h:622:10: note: in definition of macro ‘kunit_log’
> >   622 |   printk(lvl fmt, ##__VA_ARGS__);    \
> >       |          ^~~
> > ../include/kunit/test.h:641:2: note: in expansion of macro ‘kunit_printk’
> >   641 |  kunit_printk(KERN_INFO, test, fmt, ##__VA_ARGS__)
> >       |  ^~~~~~~~~~~~
> > ../include/linux/kern_levels.h:14:19: note: in expansion of macro ‘KERN_SOH’
> >    14 | #define KERN_INFO KERN_SOH "6" /* informational */
> >       |                   ^~~~~~~~
> > ../include/kunit/test.h:641:15: note: in expansion of macro ‘KERN_INFO’
> >   641 |  kunit_printk(KERN_INFO, test, fmt, ##__VA_ARGS__)
> >       |               ^~~~~~~~~
> > ../lib/kunit/kunit-example-test.c:23:2: note: in expansion of macro ‘kunit_info’
> >    23 |  kunit_info(test, "invalid: %s", 42);
> >
> > After this patch, it gets noisier:
> > In file included from ../lib/kunit/kunit-example-test.c:9:
> > ../lib/kunit/kunit-example-test.c: In function ‘example_simple_test’:
> > ../include/linux/kern_levels.h:5:18: warning: format ‘%s’ expects
> > argument of type ‘char *’, but argument 3 has type ‘int’ [-Wformat=]
> >     5 | #define KERN_SOH "\001"  /* ASCII Start Of Header */
> >       |                  ^~~~~~
> > ../include/kunit/test.h:622:10: note: in definition of macro ‘kunit_log’
> >   622 |   printk(lvl fmt, ##__VA_ARGS__);    \
> >       |          ^~~
> > ../include/kunit/test.h:641:2: note: in expansion of macro ‘kunit_printk’
> >   641 |  kunit_printk(KERN_INFO, test, fmt, ##__VA_ARGS__)
> >       |  ^~~~~~~~~~~~
> > ../include/linux/kern_levels.h:14:19: note: in expansion of macro ‘KERN_SOH’
> >    14 | #define KERN_INFO KERN_SOH "6" /* informational */
> >       |                   ^~~~~~~~
> > ../include/kunit/test.h:641:15: note: in expansion of macro ‘KERN_INFO’
> >   641 |  kunit_printk(KERN_INFO, test, fmt, ##__VA_ARGS__)
> >       |               ^~~~~~~~~
> > ../lib/kunit/kunit-example-test.c:23:2: note: in expansion of macro ‘kunit_info’
> >    23 |  kunit_info(test, "invalid: %s", 42);
> >       |  ^~~~~~~~~~
> > ../include/kunit/test.h:105:31: warning: format ‘%s’ expects argument
> > of type ‘char *’, but argument 4 has type ‘int’ [-Wformat=]
> >   105 | #define KUNIT_SUBTEST_INDENT  "    "
> >       |                               ^~~~~~
> > ../include/kunit/test.h:623:42: note: in definition of macro ‘kunit_log’
> >   623 |   kunit_log_append((test_or_suite)->log, fmt "\n", \
> >       |                                          ^~~
> > ../include/kunit/test.h:628:23: note: in expansion of macro
> > ‘KUNIT_SUBTEST_INDENT’
> >   628 |  kunit_log(lvl, test, KUNIT_SUBTEST_INDENT "# %s: " fmt,  \
> >       |                       ^~~~~~~~~~~~~~~~~~~~
> > ../include/kunit/test.h:641:2: note: in expansion of macro ‘kunit_printk’
> >   641 |  kunit_printk(KERN_INFO, test, fmt, ##__VA_ARGS__)
> >       |  ^~~~~~~~~~~~
> > ../lib/kunit/kunit-example-test.c:23:2: note: in expansion of macro ‘kunit_info’
> >    23 |  kunit_info(test, "invalid: %s", 42);
> >       |  ^~~~~~~~~~
> >
> >
>
> Yeah: that is pretty ugly. TBH, it was pretty ugly beforehand, and
> this does make it worse. I guess that's the price we pay for having so
> many nested macros, as well.
> Personally, I suspect this is still worth it to get rid of the
> compiler warnings, but only just.
>
> > >
> > >  /*
> > >   * printk and log to per-test or per-suite log buffer.  Logging only done
> > > diff --git a/lib/kunit/string-stream.h b/lib/kunit/string-stream.h
> > > index fe98a00b75a9..5e94b623454f 100644
> > > --- a/lib/kunit/string-stream.h
> > > +++ b/lib/kunit/string-stream.h
> > > @@ -35,9 +35,9 @@ struct string_stream *alloc_string_stream(struct kunit *test, gfp_t gfp);
> > >  int __printf(2, 3) string_stream_add(struct string_stream *stream,
> > >                                      const char *fmt, ...);
> > >
> > > -int string_stream_vadd(struct string_stream *stream,
> > > -                      const char *fmt,
> > > -                      va_list args);
> > > +int __printf(2, 0) string_stream_vadd(struct string_stream *stream,
> > > +                                     const char *fmt,
> > > +                                     va_list args);
> >
> > This is never called with a literal `fmt` string.
> > It's currently only ever called through the _add variant, which does
> > have __printf(2,3).
> >
> > So this can't catch any mistakes currently.
> > And I think it's hard to imagine we'd ever pass in a literal format
> > string w/ a va_list.
> >
>
> Yeah: I was tempted to leave this one out, but it was triggering
> warnings with the "you should use __printf()" heuristic. In fact, it
> had two warnings.
> The __printf() specifier documentation does specifically call out
> cases where a va_list is passed in as a case to use '0' for the
> positional argument, but only the format string is checked for
> validity: there's no typechecking.
>
> > >
> > >  char *string_stream_get_string(struct string_stream *stream);
> > >
> > > --
> > > 2.31.1.751.gd2f1c929bd-goog
> > >
> > > --
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