Re: [PATCH] Set TIF_IRET in more places

[Zack Weinberg <zack@codesourcery.com>]

Jamie Lokier <lk@tantalophile.demon.co.uk> writes:

> Zack Weinberg wrote:
>> Consider SA_RESTORER - there isn't a guarantee that user space will
>> use the same code as the kernel's trampoline.  glibc happens to, but
>> only because GDB has a hardwired idea of what a signal trampoline
>> looks like.  Of course, you could simply document that sigreturn() is
>> another of the system calls that must be made through int 0x80.
>
> Glibc must use the same code as the kernel's trampoline because of
> MD_FALLBACK_FRAME_STATE_FOR() in GCC's exception handling...  (or
> libgcc.so must change).
>
> It explicitly checks for the opcode sequences 0x58b877000000cd80 and
> 0xb8ad000000cd80 in order to unwind exception frames around a
> handled signal.  Ugly, isn't it?

We're open to better ideas ...

>> Tangentially, I've seen people claim that the trampoline ought to be
>> able to avoid entering the kernel, although I'm not convinced (how
>> does the signal mask get reset, otherwise?)
>
> Welcome to a wonderful if rather unsightly optimisation:
[...]

I would want to be very sure that this was actually a performance win
before implementing it, and since it requires data tables in user
space I don't see how it could possibly be done in the vsyscall page.

zw