Date:   Fri, 20 Dec 2019 05:52:36 +0300
From:   Vitaly Chikunov <vt@...linux.org>
To:     Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        stable@...r.kernel.org, acme@...nel.org
Subject: Re: [PATCH] tools lib: Disable redundant-delcs error for strlcpy

Arnaldo,

On Tue, Dec 17, 2019 at 05:04:20PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Dec 17, 2019 at 03:23:32PM +0300, Vitaly Chikunov escreveu:
> > Arnaldo,

(Btw, you didn't include me into the To: of your reply).

> > Ping. Can you accept or comment on this patch? There is further
> > explanations of it:
> 
> Will this work when building with clang

Clang doesn't produce this warning:

  https://clang.llvm.org/docs/DiagnosticsReference.html#wredundant-decls
  "-Wredundant-decls
  This diagnostic flag exists for GCC compatibility, and has no effect
  in Clang."

Thus, this change doesn't affect clang. (When building the kernel objtool
compiles OK).

But, compilation with clang fails compiling perf with:

    CC       util/string.o
  ../lib/string.c:99:8: error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
  size_t __weak strlcpy(char *dest, const char *src, size_t size)
         ^
  ../../tools/include/linux/compiler.h:66:34: note: expanded from macro '__weak'
  # define __weak                 __attribute__((weak))
                                                 ^
  /usr/include/bits/string_fortified.h:151:8: note: previous definition is here
  __NTH (strlcpy (char *__restrict __dest, const char *__restrict __src,
         ^
  1 error generated.

This warning could be disabled with this:

diff --git tools/lib/string.c tools/lib/string.c
index f2ae1b87c719..65b569014446 100644
--- tools/lib/string.c
+++ tools/lib/string.c
@@ -96,6 +96,8 @@ int strtobool(const char *s, bool *res)
  * If libc has strlcpy() then that version will override this
  * implementation:
  */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wignored-attributes"
 size_t __weak strlcpy(char *dest, const char *src, size_t size)
 {
        size_t ret = strlen(src);
@@ -107,6 +109,7 @@ size_t __weak strlcpy(char *dest, const char *src, size_t size)
        }
        return ret;
 }
+#pragma GCC diagnostic pop

 /**
  * skip_spaces - Removes leading whitespace from @str.

If this is acceptable I will resend v2 with this.

Thanks,

> 
> - Arnaldo
>  
> > 1. It seems that people putting strlcpy() into the tools was already aware of
> > the problems it causes and tried to solve them. Probably, that's why they put
> > `__weak` attribute on it (so it would be linkable in the presence of another
> > strlcpy). Then `#ifndef __UCLIBC__`ed and later `#if defined(__GLIBC__) &&
> > !defined(__UCLIBC__)` its declaration. But, solution was incomplete and could
> > be improved to make kernel buildable on more systems (where libc contains
> > strlcpy).
> > 
> > There is not need to make `redundant redeclaration` warning an error in
> > this case.
> > 
> > 2. `#pragma GCC diagnostic ignored` trick is already used multiple times
> > in the kernel:
> > 
> >   $ git grep  '#pragma GCC diagnostic ignored'
> >   arch/arm/lib/xor-neon.c:#pragma GCC diagnostic ignored "-Wunused-variable"
> >   tools/build/feature/test-gtk2-infobar.c:#pragma GCC diagnostic ignored "-Wstrict-prototypes"
> >   tools/build/feature/test-gtk2.c:#pragma GCC diagnostic ignored "-Wstrict-prototypes"
> >   tools/include/linux/string.h:#pragma GCC diagnostic ignored "-Wredundant-decls"
> >   tools/lib/bpf/libbpf.c:#pragma GCC diagnostic ignored "-Wformat-nonliteral"
> >   tools/perf/ui/gtk/gtk.h:#pragma GCC diagnostic ignored "-Wstrict-prototypes"
> >   tools/testing/selftests/kvm/lib/assert.c:#pragma GCC diagnostic ignored "-Wunused-result"
> >   tools/usb/ffs-test.c:#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
> > 
> > So the solution does not seem alien in the kernel and should be acceptable.
> > 
> > (I also send this to another of your emails in case I used wrong one before.)
> > 
> > Thanks,
> > 
> > 
> > On Mon, Dec 09, 2019 at 12:46:07AM +0300, Vitaly Chikunov wrote:
> > > Disable `redundant-decls' error for strlcpy declaration and solve build
> > > error allowing users to compile vanilla kernels.
> > > 
> > > When glibc have strlcpy (such as in ALT linux since 2004) objtool and
> > > perf build fails with something like:
> > > 
> > >   In file included from exec-cmd.c:3:
> > >   tools/include/linux/string.h:20:15: error: redundant redeclaration of ‘strlcpy’ [-Werror=redundant-decls]
> > >      20 | extern size_t strlcpy(char *dest, const char *src, size_t size);
> > > 	|               ^~~~~~~
> > > 
> > > It's very hard to produce a perfect fix for that since it is a header
> > > file indirectly pulled from many sources from different Makefile builds.
> > > 
> > > Fixes: ce99091 ("perf tools: Move strlcpy() from perf to tools/lib/string.c")
> > > Fixes: 0215d59 ("tools lib: Reinstate strlcpy() header guard with __UCLIBC__")
> > > Signed-off-by: Vitaly Chikunov <vt@...linux.org>
> > > Cc: Dmitry V. Levin <ldv@...linux.org>
> > > Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> > > Cc: Vineet Gupta <Vineet.Gupta1@...opsys.com>
> > > Cc: stable@...r.kernel.org
> > > ---
> > >  tools/include/linux/string.h | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/tools/include/linux/string.h b/tools/include/linux/string.h
> > > index 980cb9266718..99ede7f5dfb8 100644
> > > --- a/tools/include/linux/string.h
> > > +++ b/tools/include/linux/string.h
> > > @@ -17,7 +17,10 @@ int strtobool(const char *s, bool *res);
> > >   * However uClibc headers also define __GLIBC__ hence the hack below
> > >   */
> > >  #if defined(__GLIBC__) && !defined(__UCLIBC__)
> > > +#pragma GCC diagnostic push
> > > +#pragma GCC diagnostic ignored "-Wredundant-decls"
> > >  extern size_t strlcpy(char *dest, const char *src, size_t size);
> > > +#pragma GCC diagnostic pop
> > >  #endif
> > >  
> > >  char *str_error_r(int errnum, char *buf, size_t buflen);
> 

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