linux-kernel - Re: [PATCH v2 1/1] pgo: Fix allocate

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Date:   Thu, 3 Jun 2021 14:00:38 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Jarmo Tiitto <jarmo.tiitto@...il.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Bill Wendling <wcw@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Bill Wendling <morbo@...gle.com>
Subject: Re: [PATCH v2 1/1] pgo: Fix allocate_node() v2

On Thu, Jun 3, 2021 at 1:52 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On 6/3/2021 1:50 PM, Nick Desaulniers wrote:
> > On Thu, Jun 3, 2021 at 6:41 AM Jarmo Tiitto <jarmo.tiitto@...il.com> wrote:
> >>
> >> Based on Kees and others feedback here is v2 patch
> >> that clarifies why the current checks in allocate_node()
> >> are flawed. I did fair amount of KGDB time on it.
> >
> > Kees can probably cut it when merging, but the above paragraph might
> > be better "below the fold" below next time (doesn't necessitate a v3).
> >
> >>
> >> When clang instrumentation eventually calls allocate_node()
> >> the struct llvm_prf_data *p argument tells us from what section
> >> we should reserve the vnode: It either points into vmlinux's
> >> core __llvm_prf_data section or some loaded module's
> >> __llvm_prf_data section.
> >>
> >> But since we don't have access to corresponding
> >> __llvm_prf_vnds section(s) for any module, the function
> >> should return just NULL and ignore any profiling attempts
> >> from modules for now.
> >>
> >> Signed-off-by: Jarmo Tiitto <jarmo.tiitto@...il.com>
> >> ---
> >
> > ^ ie. here. If you put text between the `---` and the diffstat, git
> > just discards it when applying. It's a good way to hide commentary
> > just meant for reviewers when sending a patch.
> >
> >
> >>   kernel/pgo/instrument.c | 21 ++++++++++++---------
> >>   1 file changed, 12 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
> >> index 0e07ee1b17d9..afe9982b07a3 100644
> >> --- a/kernel/pgo/instrument.c
> >> +++ b/kernel/pgo/instrument.c
> >> @@ -23,6 +23,7 @@
> >>   #include <linux/export.h>
> >>   #include <linux/spinlock.h>
> >>   #include <linux/types.h>
> >> +#include <asm-generic/sections.h>
> >>   #include "pgo.h"
> >>
> >>   /*
> >> @@ -55,17 +56,19 @@ void prf_unlock(unsigned long flags)
> >>   static struct llvm_prf_value_node *allocate_node(struct llvm_prf_data *p,
> >>                                                   u32 index, u64 value)
> >>   {
> >> -       if (&__llvm_prf_vnds_start[current_node + 1] >= __llvm_prf_vnds_end)
> >> -               return NULL; /* Out of nodes */
> >> -
> >> -       current_node++;
> >> -
> >> -       /* Make sure the node is entirely within the section */
> >> -       if (&__llvm_prf_vnds_start[current_node] >= __llvm_prf_vnds_end ||
> >> -           &__llvm_prf_vnds_start[current_node + 1] > __llvm_prf_vnds_end)
> >> +       const int max_vnds = prf_vnds_count();
> >
> > Sorry, where was prf_vnds_count() defined? I don't see it in
> > linux-next, but I'm also not sure which tree has
> > 5d0cda65918279ada060417c5fecb7e86ccb3def.
>
> It is generated via the __DEFINE_PRF_SIZE macro in kernel/pgo/pgo.h.

Thanks, it kills me when I can't find something with grep. I wonder if
language servers more modern than ctags have figured this out yet.

Patch looks fine to me then.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> >> +       /* Check that p is within vmlinux __llvm_prf_data section.
> >> +        * If not, don't allocate since we can't handle modules yet.
> >> +        */
> >> +       if (!memory_contains(__llvm_prf_data_start,
> >> +               __llvm_prf_data_end, p, sizeof(*p)))
> >>                  return NULL;
> >>
> >> -       return &__llvm_prf_vnds_start[current_node];
> >> +       if (WARN_ON_ONCE(current_node >= max_vnds))
> >> +               return NULL; /* Out of nodes */
> >> +
> >> +       /* reserve vnode for vmlinux */
> >> +       return &__llvm_prf_vnds_start[current_node++];
> >>   }
> >>
> >>   /*
> >>
> >> base-commit: 5d0cda65918279ada060417c5fecb7e86ccb3def
> >> --
> >> 2.31.1
> >>
> >
> >
> > --
> > Thanks,
> > ~Nick Desaulniers
> >
>
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-- 
Thanks,
~Nick Desaulniers