On 04/08/2016 06:56 PM, Andy Lutomirski wrote:
On Fri, Apr 8, 2016 at 6:50 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote:
Hello again,
what do you think about attached patch?
I think it should fix landing problem for i386 vdso mremap.
It does not touch fast syscall path, so there should be no
speed regression.
For this thing:
+ /* Fixing userspace landing - look at do_fast_syscall_32 */
+ if (current_thread_info()->status & TS_COMPAT)
+ regs->ip = (unsigned long)current->mm->context.vdso +
+ vdso_image_32.sym_int80_landing_pad;
Either check that ip was where you expected it
And if it's not there - return error?
or simply remove this
code -- user programs that are mremapping the vdso are already playing
with fire and can just use int $0x80 to do it.
Other than that, it looks generally sane. The .mremap hook didn't
exist last time I looked at this :)
The main downside of your approach is that it doesn't allow switching
between the 32-bit, 64-bit, and x32 images. Also, it requires
awareness of how vvar and vdso line up, whereas a dedicated API could
do the whole thing.
Yes, I'm working on it. This patch will only allow moving vdso
image with general mremap - so I could use arch_prctl for
that API, as for native i386 one may move vdso with mremap
and cannot map any other vdso blobs.
Does it sound fine?
So, I have some difficulties with removing TIF_IA32 flag:
it's checked by perf for interpreting stack frames/instructions
and may be checked out of syscall executing (when tracing
page fault events, for example). I doubt, is it sane to remove
TS_COMPAT instead, leaving TIF_IA32, as for some cases
we need to know if task is compatible outside of syscall's path?
And the comment in asm/syscall.h says:
> * TIF_IA32 tasks should always have TS_COMPAT set at
> * system call time.
that means, that TS_COMPAT is always set on TIF_IA32, so
is meaningless.
What do you think?
Thanks,
Dmitry.
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