kernel-ark/arch/um
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
..
drivers uml: fix helper_wait calls in watchdog 2008-02-23 17:12:15 -08:00
include uml: fix FP register corruption 2008-02-23 17:12:15 -08:00
kernel arch/um/kernel/mem.c: fix a shadowed variable 2008-02-23 17:12:13 -08:00
os-Linux uml: fix FP register corruption 2008-02-23 17:12:15 -08:00
scripts
sys-i386 uml: remove unused sigcontext accessors 2008-02-23 17:12:15 -08:00
sys-ia64
sys-ppc uml: remove TOPDIR 2008-02-05 09:44:31 -08:00
sys-x86_64 uml: remove unused sigcontext accessors 2008-02-23 17:12:15 -08:00
defconfig UML: update defconfig 2008-02-23 17:12:13 -08:00
Kconfig uml: runtime host VMSPLIT detection 2008-02-08 09:22:42 -08:00
Kconfig.char uml: fix URLs in Kconfig and help strings 2008-02-05 09:44:25 -08:00
Kconfig.debug uml: GPROF needs to depend on FRAME_POINTER 2008-02-05 09:44:25 -08:00
Kconfig.i386 uml: runtime host VMSPLIT detection 2008-02-08 09:22:42 -08:00
Kconfig.net uml: fix URLs in Kconfig and help strings 2008-02-05 09:44:25 -08:00
Kconfig.x86_64 uml: runtime host VMSPLIT detection 2008-02-08 09:22:42 -08:00
Makefile uml: runtime host VMSPLIT detection 2008-02-08 09:22:42 -08:00
Makefile-i386
Makefile-ia64
Makefile-os-Linux
Makefile-ppc
Makefile-skas
Makefile-x86_64