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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 12 Mar 2021 13:57:47 -0800
Message-ID: <CAKwvOdmV5co+mMSBbnnXyBXiwOha=S987PMA68Xe9jP8gJYkdw@mail.gmail.com>
Subject: Re: [PATCH] gcov: fix clang-11+ support
To:     Nathan Chancellor <nathan@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc:     Peter Oberparleiter <oberpar@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Fangrui Song <maskray@google.com>,
        Prasad Sodagudi <psodagud@quicinc.com>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Fri, Mar 12, 2021 at 12:51 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Fri, Mar 12, 2021 at 12:14:42PM -0800, Nick Desaulniers wrote:
> > On Fri, Mar 12, 2021 at 11:58 AM Nathan Chancellor <nathan@kernel.org> wrote:
> > >
> > > On Fri, Mar 12, 2021 at 11:21:39AM -0800, Nick Desaulniers wrote:
> > > > LLVM changed the expected function signatures for llvm_gcda_start_file()
> > > > and llvm_gcda_emit_function() in the clang-11 release. Users of clang-11
> > > > or newer may have noticed their kernels failing to boot due to a panic
> > > > when enabling CONFIG_GCOV_KERNEL=y +CONFIG_GCOV_PROFILE_ALL=y.  Fix up
> > > > the function signatures so calling these functions doesn't panic the
> > > > kernel.
> > > >
> > > > When we drop clang-10 support from the kernel, we should carefully
> > > > update the original implementations to try to preserve git blame,
> > > > deleting these implementations.
> > > >
> > > > Link: https://reviews.llvm.org/rGcdd683b516d147925212724b09ec6fb792a40041
> > > > Link: https://reviews.llvm.org/rG13a633b438b6500ecad9e4f936ebadf3411d0f44
> > > > Cc: Fangrui Song <maskray@google.com>
> > > > Reported-by: Prasad Sodagudi<psodagud@quicinc.com>
> > > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > >
> > > I can reproduce the panic (as a boot hang) in QEMU before this patch and
> > > it is resolved after it so:
> > >
> > > Tested-by: Nathan Chancellor <nathan@kernel.org>
> > >
> > > However, the duplication hurts :( would it potentially be better to just
> > > do the full update to clang-11+ and require it for CONFIG_GCOV_KERNEL?
> > >
> > >     depends on CC_IS_GCC || CLANG_VERSION >= 110000?
> >
> > I'm not opposed, and value your input on the matter.  Either way, this
> > will need to be back ported to stable.  Should we be concerned with
> > users of stable's branches before we mandated clang-10 as the minimum
> > supported version?
> >
> > commit 1f7a44f63e6c ("compiler-clang: add build check for clang 10.0.1")
> >
> > first landed in v5.10-rc1. Does not exist in v5.4.y.  The diff you
>
> Hmmm fair point, I did not realize that this support had landed in 5.2
> meaning that 5.4 needs it as well at 5.10.
>
> > suggest is certainly easier to review to observe the differences, and
> > I we don't have users of the latest Android or CrOS kernels using
> > older clang, but I suspect there may be older kernel versions where if
> > they try to upgrade their version of clang, GCOV support will regress
> > for them.  Though, I guess that's fine since either approach will fix
> > this for them. I guess if they don't want to upgrade from clang-10 say
> > for example, then this approach can be backported to stable.
>
> If people are happy with this approach, it is the more "stable friendly"
> change because it fixes it for all versions of clang that should have
> been supported at their respective times. Maybe it is worthwhile to do
> both? This change gets picked up with a Cc: stable@vger.kernel.org then
> in a follow up patch, we remove the #ifdef's and gate GCOV on clang-11?
> The CLANG_VERSION string is usually what we will search for when
> removing old workarounds.

Sounds like we're on the same page; will send a v2 with your
recommendation on top.

> Additionally, your patch could just use
>
> #if CLANG_VERSION <= 110000
>
> to more easily see this. I have no strong opinion one way or the other
> though. If people are happy with this approach, let's do it.

Err that would be nicer, but:
kernel/gcov/clang.c:78:5: warning: 'CLANG_VERSION' is not defined,
evaluates to 0 [-Wundef]
#if CLANG_VERSION < 110000
    ^
kernel/gcov/clang.c:110:5: warning: 'CLANG_VERSION' is not defined,
evaluates to 0 [-Wundef]
#if CLANG_VERSION < 110000
    ^
kernel/gcov/clang.c:130:5: warning: 'CLANG_VERSION' is not defined,
evaluates to 0 [-Wundef]
#if CLANG_VERSION < 110000
    ^
kernel/gcov/clang.c:330:5: warning: 'CLANG_VERSION' is not defined,
evaluates to 0 [-Wundef]
#if CLANG_VERSION < 110000
    ^
kernel/gcov/clang.c:420:5: warning: 'CLANG_VERSION' is not defined,
evaluates to 0 [-Wundef]
#if CLANG_VERSION < 110000
    ^

Did we just break this in commit aec6c60a01d3 ("kbuild: check the
minimum compiler version in Kconfig") in v5.12-rc1?  So I'll keep it
as is for v2, but we should discuss with Masahiro and Miguel if we
should be removing CLANG_VERSION even if there are no in tree users at
the moment.  (I guess I could re-introduce it in my series for v2, but
that will unnecessarily complicate the backports, so I won't).  My
fault for not catching that in code review.

>
> Cheers,
> Nathan
>
> > >
> > > > ---
> > > >  kernel/gcov/clang.c | 69 +++++++++++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 69 insertions(+)
> > > >
> > > > diff --git a/kernel/gcov/clang.c b/kernel/gcov/clang.c
> > > > index c94b820a1b62..20e6760ec05d 100644
> > > > --- a/kernel/gcov/clang.c
> > > > +++ b/kernel/gcov/clang.c
> > > > @@ -75,7 +75,9 @@ struct gcov_fn_info {
> > > >
> > > >       u32 num_counters;
> > > >       u64 *counters;
> > > > +#if __clang_major__ < 11
> > > >       const char *function_name;
> > > > +#endif
> > > >  };
> > > >
> > > >  static struct gcov_info *current_info;
> > > > @@ -105,6 +107,7 @@ void llvm_gcov_init(llvm_gcov_callback writeout, llvm_gcov_callback flush)
> > > >  }
> > > >  EXPORT_SYMBOL(llvm_gcov_init);
> > > >
> > > > +#if __clang_major__ < 11
> > > >  void llvm_gcda_start_file(const char *orig_filename, const char version[4],
> > > >               u32 checksum)
> > > >  {
> > > > @@ -113,7 +116,17 @@ void llvm_gcda_start_file(const char *orig_filename, const char version[4],
> > > >       current_info->checksum = checksum;
> > > >  }
> > > >  EXPORT_SYMBOL(llvm_gcda_start_file);
> > > > +#else
> > > > +void llvm_gcda_start_file(const char *orig_filename, u32 version, u32 checksum)
> > > > +{
> > > > +     current_info->filename = orig_filename;
> > > > +     current_info->version = version;
> > > > +     current_info->checksum = checksum;
> > > > +}
> > > > +EXPORT_SYMBOL(llvm_gcda_start_file);
> > > > +#endif
> > > >
> > > > +#if __clang_major__ < 11
> > > >  void llvm_gcda_emit_function(u32 ident, const char *function_name,
> > > >               u32 func_checksum, u8 use_extra_checksum, u32 cfg_checksum)
> > > >  {
> > > > @@ -133,6 +146,24 @@ void llvm_gcda_emit_function(u32 ident, const char *function_name,
> > > >       list_add_tail(&info->head, &current_info->functions);
> > > >  }
> > > >  EXPORT_SYMBOL(llvm_gcda_emit_function);
> > > > +#else
> > > > +void llvm_gcda_emit_function(u32 ident, u32 func_checksum,
> > > > +             u8 use_extra_checksum, u32 cfg_checksum)
> > > > +{
> > > > +     struct gcov_fn_info *info = kzalloc(sizeof(*info), GFP_KERNEL);
> > > > +
> > > > +     if (!info)
> > > > +             return;
> > > > +
> > > > +     INIT_LIST_HEAD(&info->head);
> > > > +     info->ident = ident;
> > > > +     info->checksum = func_checksum;
> > > > +     info->use_extra_checksum = use_extra_checksum;
> > > > +     info->cfg_checksum = cfg_checksum;
> > > > +     list_add_tail(&info->head, &current_info->functions);
> > > > +}
> > > > +EXPORT_SYMBOL(llvm_gcda_emit_function);
> > > > +#endif
> > > >
> > > >  void llvm_gcda_emit_arcs(u32 num_counters, u64 *counters)
> > > >  {
> > > > @@ -295,6 +326,7 @@ void gcov_info_add(struct gcov_info *dst, struct gcov_info *src)
> > > >       }
> > > >  }
> > > >
> > > > +#if __clang_major__ < 11
> > > >  static struct gcov_fn_info *gcov_fn_info_dup(struct gcov_fn_info *fn)
> > > >  {
> > > >       size_t cv_size; /* counter values size */
> > > > @@ -322,6 +354,28 @@ static struct gcov_fn_info *gcov_fn_info_dup(struct gcov_fn_info *fn)
> > > >       kfree(fn_dup);
> > > >       return NULL;
> > > >  }
> > > > +#else
> > > > +static struct gcov_fn_info *gcov_fn_info_dup(struct gcov_fn_info *fn)
> > > > +{
> > > > +     size_t cv_size; /* counter values size */
> > > > +     struct gcov_fn_info *fn_dup = kmemdup(fn, sizeof(*fn),
> > > > +                     GFP_KERNEL);
> > > > +     if (!fn_dup)
> > > > +             return NULL;
> > > > +     INIT_LIST_HEAD(&fn_dup->head);
> > > > +
> > > > +     cv_size = fn->num_counters * sizeof(fn->counters[0]);
> > > > +     fn_dup->counters = vmalloc(cv_size);
> > > > +     if (!fn_dup->counters) {
> > > > +             kfree(fn_dup);
> > > > +             return NULL;
> > > > +     }
> > > > +
> > > > +     memcpy(fn_dup->counters, fn->counters, cv_size);
> > > > +
> > > > +     return fn_dup;
> > > > +}
> > > > +#endif
> > > >
> > > >  /**
> > > >   * gcov_info_dup - duplicate profiling data set
> > > > @@ -362,6 +416,7 @@ struct gcov_info *gcov_info_dup(struct gcov_info *info)
> > > >   * gcov_info_free - release memory for profiling data set duplicate
> > > >   * @info: profiling data set duplicate to free
> > > >   */
> > > > +#if __clang_major__ < 11
> > > >  void gcov_info_free(struct gcov_info *info)
> > > >  {
> > > >       struct gcov_fn_info *fn, *tmp;
> > > > @@ -375,6 +430,20 @@ void gcov_info_free(struct gcov_info *info)
> > > >       kfree(info->filename);
> > > >       kfree(info);
> > > >  }
> > > > +#else
> > > > +void gcov_info_free(struct gcov_info *info)
> > > > +{
> > > > +     struct gcov_fn_info *fn, *tmp;
> > > > +
> > > > +     list_for_each_entry_safe(fn, tmp, &info->functions, head) {
> > > > +             vfree(fn->counters);
> > > > +             list_del(&fn->head);
> > > > +             kfree(fn);
> > > > +     }
> > > > +     kfree(info->filename);
> > > > +     kfree(info);
> > > > +}
> > > > +#endif
> > > >
> > > >  #define ITER_STRIDE  PAGE_SIZE
> > > >
> > > >
> > > > base-commit: f78d76e72a4671ea52d12752d92077788b4f5d50
> > > > --
> > > > 2.31.0.rc2.261.g7f71774620-goog
> > > >
> >
> >
> >
> > --
> > Thanks,
> > ~Nick Desaulniers


-- 
Thanks,
~Nick Desaulniers