Re: [PATCH] kunit: Add gnu_printf specifiers

[David Gow]

On Fri, May 14, 2021 at 4:25 AM Daniel Latypov <dlatypov@xxxxxxxxxx> wrote:
>
> On Thu, May 13, 2021 at 1:03 PM 'David Gow' via KUnit Development
> <kunit-dev@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
>
> Reviewed-by: Daniel Latypov <dlatypov@xxxxxxxxxx>
>
> 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.


> > ---
> >  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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