Re: [PATCH] x86_64 signal handling for 64-bit apps w/ mixed 32-bit code - trivial fix

[Bryan Ford <baford@mit.edu>]

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On Thursday 06 October 2005 12:46, Andi Kleen wrote:
> Bryan Ford <baford@mit.edu> writes:
> > The proposed patch doesn't affect any performance-critical paths (e.g.,
> > syscall or interrupt entry/exit), and merely involves a couple more moves
> > to/from user space on signal frame setup and sigreturn.  It also doesn't
> > affect the size or shape of the sigcontext at all, since there already
> > was an (unused) slot for CS, and I've assigned the convenient __pad0
> > field as a slot for SS.  The existing, unused slots for FS and GS remain
> > unused for now, and I don't see any urgent need to change that.  The only
> > way this might break an existing app is if the app tries to cons up its
> > own signal frame (not generated by the kernel) and pass it to sigreturn,
> > but this is presumably a no-no anyway.
>
> I see the point of saving/restore cs, but why ss and not es/ds ?

Hmm - I remember at one point thinking there was a reason it was necessary for 
the kernel to save SS as well as CS, but now I can't remember what it was - 
and now that you mention it, it does seem unnecessary for the kernel to save 
SS since its value is irrelevant to the CPU while running the 64-bit signal 
handler code, and the 64-bit signal handler itself can save and restore it if 
necessary (as it can with ES/DS/FS/GS).

Saving SS along with CS would make the 64-bit signal path more consistent with 
the 32-bit signal path and with the way the processor's interrupt/IRET 
mechanism works, but neither of these considerations seem very important.  So 
on the assumption you'd prefer the code path to do the absolute minimum it 
can get away with, here's a new version of my previously posted patch, 
trimmed so it only saves and restores CS.  This of course makes it 
unnecessary even to rename the __pad0 field in struct sigcontext.

Thanks,
Bryan

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diff -ur orig/arch/x86_64/kernel/signal.c linux-2.6.13.3/arch/x86_64/kernel/signal.c
--- orig/arch/x86_64/kernel/signal.c	2005-10-06 16:00:54.000000000 -0400
+++ linux-2.6.13.3/arch/x86_64/kernel/signal.c	2005-10-06 16:02:23.000000000 -0400
@@ -110,6 +110,13 @@
 	COPY(r14);
 	COPY(r15);
 
+	/* Kernel saves and restores only the CS segment register on signals,
+	 * which is the bare minimum needed to allow mixed 32/64-bit code.
+	 * App's signal handler can save/restore other segments if needed. */
+	unsigned short cs;
+	err |= __get_user(cs, &sc->cs);
+	regs->cs = cs | 3;	/* Force into user mode */
+
 	{
 		unsigned int tmpflags;
 		err |= __get_user(tmpflags, &sc->eflags);
@@ -187,6 +194,7 @@
 {
 	int err = 0;
 
+	err |= __put_user(regs->cs, &sc->cs);
 	err |= __put_user(0, &sc->gs);
 	err |= __put_user(0, &sc->fs);
 
@@ -318,7 +326,14 @@
 
 	regs->rsp = (unsigned long)frame;
 
+	/* Set up the CS register to run signal handlers in 64-bit mode,
+	   even if the handler happens to be interrupting 32-bit code. */
+	regs->cs = __USER_CS;
+
+	/* This, by contrast, has nothing to do with segment registers -
+	   see include/asm-x86_64/uaccess.h for details. */
 	set_fs(USER_DS);
+
 	regs->eflags &= ~TF_MASK;
 	if (test_thread_flag(TIF_SINGLESTEP))
 		ptrace_notify(SIGTRAP);