Re: [RFC v2] x86-64: Allow emulated vsyscalls from user addresses

From: Andrew Lutomirski
Date: Thu Aug 11 2011 - 09:27:43 EST

On Thu, Aug 11, 2011 at 9:16 AM, Pavel Machek <pavel@xxxxxx> wrote:
> Hi!
>> They trace control flow through the vsyscall page and recompile that
>> code somewhere else.  Then they expect it to work.  DynamoRIO
>> ( and Pin ( are
>> affected.  They crash when tracing programs that use vsyscalls.
>> Valgrind is smart enough not to cause problems.  It crashes on the
>> getcpu vsyscall, but that has nothing to do with emulation.
>> This patch makes each of the three vsyscall entries use a different
>> vector so that they can work when relocated.  It assumes that the
>> code that relocates them is okay with the int instruction acting
>> like ret.  DynamoRIO at least appears to work.
> int acting as ret is seriously weird semantics. And no, invalid
> syscall parameters will not cause segfault, just return of -EFAULT. So
> ... can this be changed?

Can which be changed? int acting as ret already was (in a different
patch, now in tip/x86/vdso), although I still think that user code
should do its best to make no assumptions about the vsyscall page.

invalid syscall parameters do indeed return -EFAULT, but invalid
*v*syscall parameters will segfault in 3.0 and before, since the
vsyscall implementation is just user code and has no exception

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