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

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Date:   Thu, 3 Jun 2021 13:52:43 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Jarmo Tiitto <jarmo.tiitto@...il.com>
Cc:     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 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.

>> +       /* 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
>