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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 4 Jun 2021 10:28:45 -0700
Message-ID: <CAKwvOd=4yRVZGCK36M-X_9TSxfJNWBWKfVnWO4-MQr47YkMPPA@mail.gmail.com>
Subject: Re: [PATCH v3 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 Fri, Jun 4, 2021 at 10:03 AM Jarmo Tiitto <jarmo.tiitto@gmail.com> wrote:
>
> 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>
> ---
> Based on Kees and others feedback here is v3 patch
> that clarifies why the current checks in allocate_node()
> are flawed. I did fair amount of KGDB time on it.
>
> The commit is based on kees/for-next/clang/features tree,
> hopefully this is ok. Should I have based it on linux-next
> instead?
>
> I grep -R'd where the memory_contains() can be found and it is only
> found in #include <asm-generic/sections.h>
>
> I cross my fingers and await if this is my first accepted patch. :-)
> ---
>  kernel/pgo/instrument.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
> index 0e07ee1b17d9..c69b4f7ebaad 100644
> --- a/kernel/pgo/instrument.c
> +++ b/kernel/pgo/instrument.c
> @@ -18,6 +18,7 @@
>
>  #define pr_fmt(fmt)    "pgo: " fmt
>
> +#include <asm-generic/sections.h>

^ What about Nathan's feedback on this being just `<asm/sections.h>`?

>  #include <linux/bitops.h>
>  #include <linux/kernel.h>
>  #include <linux/export.h>
> @@ -55,17 +56,21 @@ 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();
> +
> +       /*
> +        * 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