[lkml]   [2012]   [Jan]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Compat 32-bit syscall entry from 64-bit task!?
On Saturday 21 January 2012 00:49, Indan Zupancic wrote:
> On Fri, January 20, 2012 23:41, H. Peter Anvin wrote:
> > On 01/20/2012 02:40 PM, Roland McGrath wrote:
> >> If you change the size of a regset, then the new full size will be the size
> >> of the core file notes. Existing userland tools will not be expecting
> >> this, they expect a known exact size. If you need to add new stuff, it
> >> really is easier all around to add a new regset flavor. When adding a new
> >> one, you can make it variable-sized from the start so as to be extensible
> >> in the future. We did this for NT_X86_XSTATE, for example.
> >>
> >
> > Yes, that definitely seems cleaner.
> I would prefer Linus' way of just stuffing it into cs. Jamie also wanted
> a bit telling in what mode the userspace is running. That's 3 bits in total,
> with one bit telling whether the other bits are valid or not. Anything else?

There is actually a bunch of ptrace-specific stuff we want to return.

For example, Oleg wants to be able to print *which syscall*,
(along with its arguments if possible) is restarted when
we restart the ERESTART_RESTARTBLOCK-returning syscall.
Which happens every time strace attaches to a process sleeping
in nanosleep or poll, for example. We get just

$ strace -p 1234
Process 1234 attached - interrupt to quit
restart_syscall(<... resuming interrupted call ...>_

and that's it.

Returning syscall and its parameters require several words,
not a few bits.

> Maybe a bit telling whether it is syscall entry or exit?

Yes, this one too. This is one of longstanding annoyances
that this information is not exposed.

> As all this is very x86_64 specific and cs is already used to figure out
> the mode, it seems overkill to add a new regset just for this.
> It's a lot easier for existing code to add an extra cs check than to use
> different register sets and different ptrace commands.

You don't understand. Returning new bits in cs will break *existing*
programs. This is generally a bad thing. For example, old strace binaries
on new kernel will complain:

switch (x86_64_regs.cs) {
case 0x23: currpers = 1; break;
case 0x33: currpers = 0; break;
fprintf(stderr, "Unknown value CS=0x%08X while "
"detecting personality of process "
"PID=%d\n", (int)x86_64_regs.cs, tcp->pid);
currpers = current_personality;

when they'll see unfamiliar x86_64_regs.cs value.


 \ /
  Last update: 2012-01-21 01:09    [W:0.183 / U:4.884 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site