[uml-devel] [PATCH] uml: fix signal code x86-64 [for 2.6.15]

[Paolo 'B. G. <bla...@ya...>]

From: Paolo 'Blaisorblade' Giarrusso <bla...@ya...>

The problems in this area came to light while fixing a compile failure wi=
th
GCC 4, in commit bcb01b8a67476e6f748086e626df8424cc27036d. I went compari=
ng this
code with x86_64 frame construction (which we should ABI compatible with)=
 and
resync'ed the code a bit.

It isn't yet perfect, because we don't yet save floating point context. B=
ut that
will come later. Additionally, there's a potential problem since RED zone=
s will
alternate stacks are used, unlike x86_64, so more stack space (128 bytes =
more)
is used. But this shouldn't be a problem.
Instead, having no red zone (like x86_64) will cause problems when a new =
signal
is delivered on the same alternate stack as one which is being handled, (=
since
GCC will follow the ABI and place data in the red zone in the handler for=
 the
first delivered signal).

Please give a critical eye, even because things currently have no reporte=
d
misbehaviour, and this code is complex enough.

CC: Andi Kleen <ak...@su...>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <bla...@ya...>
---

 arch/um/sys-x86_64/signal.c |   19 ++++++++++++++++---
 1 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/um/sys-x86_64/signal.c b/arch/um/sys-x86_64/signal.c
index a4c46a8..7e3952a 100644
--- a/arch/um/sys-x86_64/signal.c
+++ b/arch/um/sys-x86_64/signal.c
@@ -176,6 +176,7 @@ struct rt_sigframe
=20
 #define round_down(m, n) (((m) / (n)) * (n))
=20
+/* Taken from arch/x86_64/kernel/signal.c:setup_rt_frame(). */
 int setup_signal_stack_si(unsigned long stack_top, int sig,
 			  struct k_sigaction *ka, struct pt_regs * regs,
 			  siginfo_t *info, sigset_t *set)
@@ -186,9 +187,21 @@ int setup_signal_stack_si(unsigned long=20
 	int err =3D 0;
 	struct task_struct *me =3D current;
=20
-	frame =3D (struct rt_sigframe __user *)
-		round_down(stack_top - sizeof(struct rt_sigframe), 16) - 8;
-        frame =3D (struct rt_sigframe *) ((unsigned long) frame - 128);
+	/* Leave space on the stack for the Red Zone, and for saving FP
+	 * registers, even if this doesn't happen. We don't have a way to test
+	 * used_math(), so we do that inconditionally.
+	 *
+	 * XXX: RED-PEN: currently, we're using a Red Zone also for any
+	 * alternate stack set up by sigaltstack(), which x86-64 doesn't do
+	 * (because there shouldn't be any code executing there). This could
+	 * cause failures if user setup a too little alternate stack.*/
+
+        fp =3D (struct _fpstate *) round_down(stack_top - 128 -
+				sizeof(struct _fpstate), 16);
+
+	/* Now leave the space for the rest of signal frame. */
+	frame =3D (void __user *) round_down((unsigned long) fp -
+			sizeof(struct rt_sigframe), 16) - 8;
=20
 	if (!access_ok(VERIFY_WRITE, fp, sizeof(struct _fpstate)))
 		goto out;