linux/arch/um/include/sysdep-x86_64
Jeff Dike 2f56debd77 uml: fix FP register corruption
Commit ee3d9bd4de ("uml: simplify SIGSEGV
handling"), while greatly simplifying the kernel SIGSEGV handler that
runs in the process address space, introduced a bug which corrupts FP
state in the process.

Previously, the SIGSEGV handler called the sigreturn system call by hand - it
couldn't return through the restorer provided to it because that could try to
call the libc restorer which likely wouldn't exist in the process address
space.  So, it blocked off some signals, including SIGUSR1, on entry to the
SIGSEGV handler, queued a SIGUSR1 to itself, and invoked sigreturn.  The
SIGUSR1 was delivered, and was visible to the UML kernel after sigreturn
finished.

The commit eliminated the signal masking and the call to sigreturn.  The
handler simply hits itself with a SIGTRAP to let the UML kernel know that it
is finished.  UML then restores the process registers, which effectively
longjmps the process out of the signal handler, skipping sigreturn's restoring
of register state and the signal mask.

The bug is that the host apparently sets used_fp to 0 when it saves the
process FP state in the sigcontext on the process signal stack.  Thus, when
the process is longjmped out of the handler, its FP state is corrupt because
it wasn't saved on the context switch to the UML kernel.

This manifested itself as sleep hanging.  For some reason, sleep uses floating
point in order to calculate the sleep interval.  When a page fault corrupts
its FP state, it is faked into essentially sleeping forever.

This patch saves the FP state before entering the SIGSEGV handler and restores
it afterwards.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-02-23 17:12:15 -08:00
..
archsetjmp.h fix file specification in comments 2007-05-09 08:58:16 +02:00
barrier.h [PATCH] uml: fix I/O hang 2006-11-03 12:27:58 -08:00
checksum.h [NET]: UML checksum annotations and cleanups. 2006-12-02 21:23:12 -08:00
faultinfo.h [PATCH] uml: S390 preparation, abstract host page fault data 2005-05-05 16:36:36 -07:00
kernel-offsets.h uml: redo the calculation of NR_syscalls 2008-02-05 09:44:32 -08:00
ptrace_user.h uml: fix FP register corruption 2008-02-23 17:12:15 -08:00
ptrace.h uml: eliminate floating point state from register file 2007-10-16 09:43:07 -07:00
sc.h [PATCH] uml: fix missing x86_64 register definitions 2006-09-27 08:26:16 -07:00
sigcontext.h uml: style fixes in FP code 2007-10-16 09:43:07 -07:00
skas_ptrace.h [PATCH] uml: S390 preparation, abstract host page fault data 2005-05-05 16:36:36 -07:00
stub.h uml: fix stub address calculations 2007-10-16 09:43:09 -07:00
syscalls.h uml: redo the calculation of NR_syscalls 2008-02-05 09:44:32 -08:00
tls.h [PATCH] uml: tls support: hack to make it compile on any host 2006-03-31 12:18:52 -08:00