[PATCH] x86_64: Save/restore CS in 64bit signal handlers and force __USER_CS for CS

This allows to run 64bit signal handlers in 64bit processes that run small
code snippets in compat mode.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/arch/x86_64/kernel/signal.c b/arch/x86_64/kernel/signal.c
index d642fbf3da29d7823cd2a22f4329858aad7d76f3..5876df116c9225ef449e58a3d06e3de9b66c6445 100644 (file)
--- a/arch/x86_64/kernel/signal.c
+++ b/arch/x86_64/kernel/signal.c
@@ -110,6 +110,15 @@ restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc, unsigned
        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 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 +196,7 @@ setup_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs, unsigned lo
 {
        int err = 0;
 
+       err |= __put_user(regs->cs, &sc->cs);
        err |= __put_user(0, &sc->gs);
        err |= __put_user(0, &sc->fs);
 
@@ -318,7 +328,14 @@ static int setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
 
        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);