Re: [gdb/build] Fix Werror=nonnull-compare build breaker with gcc 12

[Andrew Burgess <andrew.burgess@embecosm.com>]

* Tom de Vries <tdevries@suse.de> [2021-07-27 18:28:30 +0200]:

> [ was : Re: Building with recent GCC versions:
> gdbsupport/gdb_assert.h:35:4: error: 'nonnull' argument 'filename'
> compared to NULL [-Werror=nonnull-compare] ]
> 
> [ Moving discussion to gdb-patches ]
> 
> On 7/27/21 3:38 PM, Tom de Vries wrote:
> > On 7/27/21 1:49 PM, Tom de Vries wrote:
> >> On 7/27/21 1:35 PM, Andrew Burgess wrote:
> >>> * Tom de Vries <tdevries@suse.de> [2021-07-27 12:44:10 +0200]:
> >>>
> >>>> On 7/27/21 12:03 PM, Andrew Burgess wrote:
> >>>>> * Jan-Benedict Glaw <jbglaw@lug-owl.de> [2021-07-26 23:11:01 +0200]:
> >>>>>
> >>>>>> Hi!
> >>>>>>
> >>>>>> I'm running some CI builds and noticed that, when building GDB with
> >>>>>> quite recent GCC versions, it breaks.
> >>>>>>
> >>>>>> With ie. this "gcc-snapshot" GCC from Debian:
> >>>>>>
> >>>>>> /usr/lib/gcc-snapshot/bin/gcc --version
> >>>>>> gcc (Debian 20210630-1) 12.0.0 20210630 (experimental) [master revision 6bf383c37e6:93c270320bb:35da8a98026849bd20d16bbf9210ac1d0b44ea6a]
> >>>>>>
> >>>>>> we see:
> >>>>>>
> >>>>>> ./configure --target=i686-linux --prefix=/tmp/gdb-i686-linux
> >>>>>> [...]
> >>>>>> all make V=1 all-gdb
> >>>>>> [...]
> >>>>>> [all 2021-07-26 20:39:22] /usr/lib/gcc-snapshot/bin/g++ -x c++    -I. -I. -I./config -DLOCALEDIR="\"/tmp/gdb-i686-linux/share/locale\"" -DHAVE_CONFIG_H -I./../include/opcode -I./../readline/readline/.. -I./../zlib -I../bfd -I./../bfd -I./../include -I../libdecnumber -I./../libdecnumber  -I./../gnulib/import -I../gnulib/import -I./.. -I..  -DTUI=1    -I./.. -pthread  -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-variable -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable -Wno-sign-compare -Wno-error=maybe-uninitialized -Wno-mismatched-tags -Wsuggest-override -Wimplicit-fallthrough=3 -Wduplicated-cond -Wshadow=local -Wdeprecated-copy -Wdeprecated-copy-dtor -Wredundant-move -Wmissing-declarations -Wstrict-null-sentinel -Wformat -Wformat-nonliteral -Werror -g -O2   -c -o compile/compile.o -MT compile/compile.o -MMD -MP -MF compile/.deps/compile.Tpo compile/compile.c
> >>>>>> [all 2021-07-26 20:39:26] In file included from ./../gdbsupport/common-defs.h:126,
> >>>>>> [all 2021-07-26 20:39:26]                  from ./defs.h:28,
> >>>>>> [all 2021-07-26 20:39:26]                  from compile/compile.c:20:
> >>>>>> [all 2021-07-26 20:39:26] ./../gdbsupport/gdb_unlinker.h: In constructor 'gdb::unlinker::unlinker(const char*)':
> >>>>>> [all 2021-07-26 20:39:26] ./../gdbsupport/gdb_assert.h:35:4: error: 'nonnull' argument 'filename' compared to NULL [-Werror=nonnull-compare]
> >>>>>> [all 2021-07-26 20:39:26]    35 |   ((void) ((expr) ? 0 :                                                       \
> >>>>>> [all 2021-07-26 20:39:26]       |   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>>>> [all 2021-07-26 20:39:26]    36 |            (gdb_assert_fail (#expr, __FILE__, __LINE__, FUNCTION_NAME), 0)))
> >>>>>> [all 2021-07-26 20:39:26]       |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>>>> [all 2021-07-26 20:39:26] ./../gdbsupport/gdb_unlinker.h:38:5: note: in expansion of macro 'gdb_assert'
> >>>>>> [all 2021-07-26 20:39:26]    38 |     gdb_assert (filename != NULL);
> >>>>>> [all 2021-07-26 20:39:26]       |     ^~~~~~~~~~
> >>>>>> [all 2021-07-26 20:39:27] cc1plus: all warnings being treated as errors
> >>>>>> [all 2021-07-26 20:39:27] make[1]: *** [Makefile:1642: compile/compile.o] Error 1
> >>>>>> [all 2021-07-26 20:39:27] make[1]: Leaving directory '/var/lib/laminar/run/gdb-i686-linux/4/binutils-gdb/gdb'
> >>>>>> [all 2021-07-26 20:39:27] make: *** [Makefile:11410: all-gdb] Error 2
> >>>>>>
> >>>>>>
> >>>>>> I also discussed this on the GCC patches mailing list
> >>>>>> (https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575568.html),
> >>>>>> where Martin suggested that this should be fixed here in GDB.
> >>>>>>
> >>>>>> Any thoughts about this?
> >>>>>
> >>>>> As I understand it the nonnull attribute only provides compile time
> >>>>> protection against explicitly passing NULL, there's no compiled in
> >>>>> non-null check (well, maybe with -fisolate-erroneous-paths-attribute,
> >>>>> but the assert might give a better error message).
> >>>>>
> >>>>> This means its still possible to pass NULL to a nonnull function, its
> >>>>> just the behaviour of the program is undefined in that case.
> >>>>>
> >>>>> So, it doesn't seem crazy that we might want to both (a) have a
> >>>>> function declared nonnull, to prevent explicitly passing NULL, and (b)
> >>>>> have a NULL check inside the function to catch logic bugs that result
> >>>>> in NULL being passed.
> >>>>>
> >>>>> We could, of course, push the assert outside of the function, but that
> >>>>> would really suck due to code duplication, and the risk of missing an
> >>>>> assert, so that seems like a non-starter.
> >>>>>
> >>>>> We could drop either the assert, or the nonnull attribute, but that
> >>>>> would suck as both give a valuable, but different form of protection.
> >>>>>
> >>>>> After some experimenting, I suspect that the assert is being optimised
> >>>>> away anyway, which kind of makes sense, as we're telling the compiler
> >>>>> it can assume that the pointer is non-null.
> >>>>>
> >>>>
> >>>> Yes, in fact that's what the nonnull-compare warning specifically warns
> >>>> against: there's some code that may be optimized away, due to the
> >>>> nonnull attribute.
> >>>>
> >>>>> So, what we probably want is someway to tell (or trick) GCC into
> >>>>> including the null check even in the nonnull function....
> >>>>>
> >>>>> ... here's what I came up with, add this somewhere:
> >>>>>
> >>>>>  template<typename T>
> >>>>>  bool __attribute__ ((noinline))
> >>>>>  nonnull_arg_is_really_not_null (const T *ptr)
> >>>>>  {
> >>>>>    return ptr != nullptr;
> >>>>>  }
> >>>>>
> >>>>> then change the assert to:
> >>>>>
> >>>>>  gdb_assert (nonnull_arg_is_really_not_null (filename));
> >>>>>
> >>>>> Seems to keep the assert, and silence the warning.  Thoughts?
> >>>>>
> >>>>
> >>>> I understand why it works, but it seems fragile to me.  At some point
> >>>> some compiler may get smart enough to also optimize this case, and then
> >>>> we're back in the same situation.
> >>>
> >>> Good point.
> >>>
> >>> The GCC documentation for noinline[1] suggests we can avoid the call
> >>> being removed by adding 'asm ("");' into the function:
> >>>
> >>>   template<typename T>
> >>>   bool __attribute__ ((noinline))
> >>>   nonnull_arg_is_really_not_null (const T *ptr)
> >>>   {
> >>>     asm ("");
> >>>     return ptr != nullptr;
> >>>   }
> >>>
> >>> I'm not really arguing for this approach over any other, just sharing
> >>> what I discovered.
> >>>
> >>
> >> Ack, understood.  Note that the added asm doesn't stop a compiler from
> >> doing:
> >> ...
> >> gdb_assert (nonnull_arg_is_really_not_null (filename));
> >> ...
> >> ->
> >> ...
> >> nonnull_arg_is_really_not_null (filename);
> >> gdb_assert (true);
> >> ...
> >>
> >> Thanks,
> >> - Tom
> >>
> >>> Thanks,
> >>> Andrew
> >>>
> >>> [1] https://gcc.gnu.org/onlinedocs/gcc-11.1.0/gcc/Common-Function-Attributes.html#Common-Function-Attributes
> >>>
> >>>>
> >>>> I wonder whether using volatile is a better idea (can't try this out
> >>>> right now).
> >>>>
> > 
> > I was thinking of something like this:
> > ...
> > diff --git a/gdbsupport/gdb_unlinker.h b/gdbsupport/gdb_unlinker.h
> > index bda6fe7ab54..3d99b41e7ad 100644
> > --- a/gdbsupport/gdb_unlinker.h
> > +++ b/gdbsupport/gdb_unlinker.h
> > @@ -20,6 +20,13 @@
> >  #ifndef COMMON_GDB_UNLINKER_H
> >  #define COMMON_GDB_UNLINKER_H
> > 
> > +template<typename T>
> > +const T *volatile
> > +ignore_nonnull (const T *ptr)
> > +{
> > +  return ptr;
> > +}
> > +
> >  namespace gdb
> >  {
> > 
> > @@ -35,7 +42,7 @@ class unlinker
> >    unlinker (const char *filename) ATTRIBUTE_NONNULL (2)
> >      : m_filename (filename)
> >    {
> > -    gdb_assert (filename != NULL);
> > +    gdb_assert (ignore_nonnull (filename) != NULL);
> >    }
> > 
> >    ~unlinker ()
> > ...
> > 
> > This builds for me, though I haven't got a setup yet where the warning
> > reproduces, so I can't check whether it actually fixes things.
> 
> I managed now to reproduce, and wrote a patch along these lines.
> 
> Any comments?
> 
> In particular, any suggestion where to put ignore_nonnull?
> 
> Or, is it perhaps a better idea to have a gdb_assert_nonnull and
> implement things there?
> 
> Thanks,
> - Tom

> [gdb/build] Fix Werror=nonnull-compare build breaker with gcc 12
> 
> When building gdb using current gcc trunk, we run into:
> ...
> gdbsupport/gdb_unlinker.h: \
>   In constructor 'gdb::unlinker::unlinker(const char*)':
> gdbsupport/gdb_assert.h:35:4: error: \
>   'nonnull' argument 'filename' compared to NULL [-Werror=nonnull-compare]
> 35| ((void) ((expr) ? 0 :                                                    \
>   | ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 36|          (gdb_assert_fail (#expr, __FILE__, __LINE__, FUNCTION_NAME), 0)))
>   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> gdbsupport/gdb_unlinker.h:38:5: note: in expansion of macro 'gdb_assert'
> 38|   gdb_assert (filename != NULL);
>   |   ^~~~~~~~~~
> cc1plus: all warnings being treated as errors
> make[1]: *** [compile/compile.o] Error 1
> ...
> 
> The warning triggers in this code:
> ...
>   unlinker (const char *filename) ATTRIBUTE_NONNULL (2)
>     : m_filename (filename)
>   {
>     gdb_assert (filename != NULL);
>   }
> ...
> 
> The attribute nonnull (applied here to the filename parameter) has two
> effects:
> - if the compiler determines that a null pointer is passed in argument
>   filename, and the -Wnonnull option is enabled, a warning is issued.
> - the compiler may perform optimizations based on the knowledge that
>   filename != NULL (unless disabled by the -fno-delete-null-pointer-checks
>   option).
> 
> The warning Werror=nonnull-compare warns that the compiler may perform the
> optimization:
> ....
>     gdb_assert (filename != NULL);
> ...
> ->
> ...
>     gdb_assert (true);
> ...
> in which case "unlinker (obfuscated_NULL)" no longer will trigger the assert.
> And we want to keep the gdb_assert to detect cases that -Wnonnull doesn't
> detect.
> 
> We could simply fix this by dropping the attribute, but that means that we no
> longer get the -Wnonnull warning.
> 
> Fix this by ignoring the nonnull attribute using a function:
> ...
> template<typename T>
> const T *volatile
> ignore_nonnull (const T *ptr)
> {
>   return ptr;
> }
> ...
> such that we can do:
> ...
>     gdb_assert (ignore_nonnull (filename) != NULL);
> ...
> 
> Build on x86_64-linux using "gcc version 12.0.0 20210727 (experimental) (GCC)"
> build from gcc commit 7ffba77d01a.
> 
> Reported-By: Jan-Benedict Glaw <jbglaw@lug-owl.de>
> 
> gdb/ChangeLog:
> 
> 2021-07-27  Andrew Burgess  <andrew.burgess@embecosm.com>
> 	    Tom de Vries  <tdevries@suse.de>
> 
> 	* gdbsupport/gdb_assert.h (ignore_nonnull): New function.
> 	* gdb/gdb_regex.c (compiled_regex::compiled_regex): Use ignore_nonnull
> 	to ignore nonnull attribute on function parameter.
> 	* gdbsupport/gdb_unlinker.h (unlinker::unlinker): Same.
> 
> ---
>  gdb/gdb_regex.c           | 4 ++--
>  gdbsupport/gdb_assert.h   | 6 ++++++
>  gdbsupport/gdb_unlinker.h | 2 +-
>  3 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/gdb/gdb_regex.c b/gdb/gdb_regex.c
> index 17fbfd28ee8..22473f57e26 100644
> --- a/gdb/gdb_regex.c
> +++ b/gdb/gdb_regex.c
> @@ -22,8 +22,8 @@
>  compiled_regex::compiled_regex (const char *regex, int cflags,
>  				const char *message)
>  {
> -  gdb_assert (regex != NULL);
> -  gdb_assert (message != NULL);
> +  gdb_assert (ignore_nonnull (regex) != NULL);
> +  gdb_assert (ignore_nonnull (message) != NULL);
>  
>    int code = regcomp (&m_pattern, regex, cflags);
>    if (code != 0)
> diff --git a/gdbsupport/gdb_assert.h b/gdbsupport/gdb_assert.h
> index 00553a78613..26a9594e7f8 100644
> --- a/gdbsupport/gdb_assert.h
> +++ b/gdbsupport/gdb_assert.h
> @@ -58,4 +58,10 @@
>    internal_error (__FILE__, __LINE__, _(message))
>  #endif
>  
> +template<typename T>
> +const T *volatile
> +ignore_nonnull (const T *ptr)
> +{
> +  return ptr;
> +}

Sorry, I missed that the thread had moved to the patches list.

Using trunk GCC I see a warning like this:

  error: 'volatile'-qualified return type is deprecated [-Werror=volatile]
     21 | const T * volatile
        | ^~~~~
  cc1plus: all warnings being treated as errors
  Compiler returned: 1

See: https://godbolt.org/z/nfhq6zb7q

Thanks,
Andrew

>  #endif /* COMMON_GDB_ASSERT_H */
> diff --git a/gdbsupport/gdb_unlinker.h b/gdbsupport/gdb_unlinker.h
> index bda6fe7ab54..4aa4f3fc3e9 100644
> --- a/gdbsupport/gdb_unlinker.h
> +++ b/gdbsupport/gdb_unlinker.h
> @@ -35,7 +35,7 @@ class unlinker
>    unlinker (const char *filename) ATTRIBUTE_NONNULL (2)
>      : m_filename (filename)
>    {
> -    gdb_assert (filename != NULL);
> +    gdb_assert (ignore_nonnull (filename) != NULL);
>    }
>  
>    ~unlinker ()