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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 3 Jun 2021 13:50:17 -0700
Message-ID: <CAKwvOd=ygrySyMkJuGwyG7OPCOrTagcFn02RfEKANvhhuZJdOA@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] pgo: Fix allocate_node() v2
To:     Jarmo Tiitto <jarmo.tiitto@gmail.com>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Bill Wendling <wcw@google.com>,
        Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <nathan@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Bill Wendling <morbo@google.com>
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On Thu, Jun 3, 2021 at 6:41 AM Jarmo Tiitto <jarmo.tiitto@gmail.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@gmail.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.

> +       /* 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