[PATCH] x86_64 ia32 syscall restart fix

From: Roland McGrath
Date: Thu Feb 28 2008 - 22:58:59 EST



The code to restart syscalls after signals depends on checking for a
negative orig_ax, and for particular negative -ERESTART* values in ax.
These fields are 64 bits and for a 32-bit task they get zero-extended.
The syscall restart behavior is lost, a regression from a native 32-bit
kernel and from 64-bit tasks' behavior. This patch fixes the problem by
doing sign-extension where it matters. For orig_ax, the only time the
value should be -1 but winds up as 0x0ffffffff is via a 32-bit ptrace
call. So the patch changes ptrace to sign-extend the 32-bit orig_eax
value when it's stored; it doesn't change the checks on orig_ax, though
it uses the new current_syscall() inline to better document the subtle
importance of the used of signedness there. The ax value is stored a
lot of ways and it seems hard to get them all sign-extended at their
origins. So for that, we use the current_syscall_ret() to sign-extend
it only for 32-bit tasks at the time of the -ERESTART* comparisons.

Signed-off-by: Roland McGrath <roland@xxxxxxxxxx>
---
arch/x86/kernel/ptrace.c | 9 ++++++++-
arch/x86/kernel/signal_64.c | 38 +++++++++++++++++++++++++++++++++-----
2 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index d862e39..3975351 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -1035,10 +1035,17 @@ static int putreg32(struct task_struct *child, unsigned regno, u32 value)
R32(esi, si);
R32(ebp, bp);
R32(eax, ax);
- R32(orig_eax, orig_ax);
R32(eip, ip);
R32(esp, sp);

+ case offsetof(struct user32, regs.orig_eax):
+ /*
+ * Sign-extend the value so that orig_eax = -1
+ * causes (long)orig_ax < 0 tests to fire correctly.
+ */
+ regs->orig_ax = (long) (s32) value;
+ break;
+
case offsetof(struct user32, regs.eflags):
return set_flags(child, value);

diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index 7347bb1..ce317ee 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -311,6 +311,35 @@ give_sigsegv:
}

/*
+ * Return -1L or the syscall number that @regs is executing.
+ */
+static long current_syscall(struct pt_regs *regs)
+{
+ /*
+ * We always sign-extend a -1 value being set here,
+ * so this is always either -1L or a syscall number.
+ */
+ return regs->orig_ax;
+}
+
+/*
+ * Return a value that is -EFOO if the system call in @regs->orig_ax
+ * returned an error. This only works for @regs from @current.
+ */
+static long current_syscall_ret(struct pt_regs *regs)
+{
+#ifdef CONFIG_IA32_EMULATION
+ if (test_thread_flag(TIF_IA32))
+ /*
+ * Sign-extend the value so (int)-EFOO becomes (long)-EFOO
+ * and will match correctly in comparisons.
+ */
+ return (int) regs->ax;
+#endif
+ return regs->ax;
+}
+
+/*
* OK, we're invoking a handler
*/

@@ -327,9 +356,9 @@ handle_signal(unsigned long sig, siginfo_t *info, struct k_sigaction *ka,
#endif

/* Are we from a system call? */
- if ((long)regs->orig_ax >= 0) {
+ if (current_syscall(regs) >= 0) {
/* If so, check system call restarting.. */
- switch (regs->ax) {
+ switch (current_syscall_ret(regs)) {
case -ERESTART_RESTARTBLOCK:
case -ERESTARTNOHAND:
regs->ax = -EINTR;
@@ -426,10 +455,9 @@ static void do_signal(struct pt_regs *regs)
}

/* Did we come from a system call? */
- if ((long)regs->orig_ax >= 0) {
+ if (current_syscall(regs) >= 0) {
/* Restart the system call - no handlers present */
- long res = regs->ax;
- switch (res) {
+ switch (current_syscall_ret(regs)) {
case -ERESTARTNOHAND:
case -ERESTARTSYS:
case -ERESTARTNOINTR:
--
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