Re: [RFC 1/6] vdso/extable: fix calculation of base

[Nadav Amit <nadav.amit@gmail.com>]

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> On Feb 25, 2021, at 1:16 PM, Sean Christopherson <seanjc@google.com> wrote:
> 
> On Wed, Feb 24, 2021, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> Apparently, the assembly considers __ex_table as the location when the
>> pushsection directive was issued. Therefore when there is more than a
>> single entry in the vDSO exception table, the calculations of the base
>> and fixup are wrong.
>> 
>> Fix the calculations of the expected fault IP and new IP by adjusting
>> the base after each entry.
>> 
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Sean Christopherson <seanjc@google.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: x86@kernel.org
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> ---
>> arch/x86/entry/vdso/extable.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/entry/vdso/extable.c b/arch/x86/entry/vdso/extable.c
>> index afcf5b65beef..c81e78636220 100644
>> --- a/arch/x86/entry/vdso/extable.c
>> +++ b/arch/x86/entry/vdso/extable.c
>> @@ -32,7 +32,7 @@ bool fixup_vdso_exception(struct pt_regs *regs, int trapnr,
>> 	nr_entries = image->extable_len / (sizeof(*extable));
>> 	extable = image->extable;
>> 
>> -	for (i = 0; i < nr_entries; i++) {
>> +	for (i = 0; i < nr_entries; i++, base += sizeof(*extable)) {
> 
> It's been literally years since I wrote this code, but I distinctly remember the
> addresses being relative to the base.  I also remember testing multiple entries,
> but again, that was a long time ago.
> 
> Assuming things have changed, or I was flat out wrong, the comment above the
> macro magic should also be updated.
> 
> /*
> * Inject exception fixup for vDSO code.  Unlike normal exception fixup,
> * vDSO uses a dedicated handler the addresses are relative to the overall
> * exception table, not each individual entry.
> */

I will update the comment. I am not very familiar with pushsection stuff,
but the offsets were wrong.

Since you say you checked it, I wonder whether it can somehow be caused
by having exception table entries defined from multiple object files.

Anyhow, this change follows the kernel’s (not vDSO) exception table
scheme.


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