[PATCH 4.1 37/78] Revert x86 sigcontext cleanups

Greg Kroah-Hartman
2015-09-11T23:03:51Z

From: Linus Torvalds <torvalds@linux-foundation.org>

commit ed596cde9425509ec6ce88e19f03e9b13b6f518b upstream.

This reverts commits 9a036b93a344 ("x86/signal/64: Remove 'fs' and 'gs'
from sigcontext") and c6f2062935c8 ("x86/signal/64: Fix SS handling for
signals delivered to 64-bit programs").

They were cleanups, but they break dosemu by changing the signal return
behavior (and removing 'fs' and 'gs' from the sigcontext struct - while
not actually changing any behavior - causes build problems).

Reported-and-tested-by: Stas Sergeev <stsp@list.ru>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/include/asm/sigcontext.h | 6 +++---
arch/x86/include/uapi/asm/sigcontext.h | 21 +++------------------
arch/x86/kernel/signal.c | 26 +++++++++++---------------
3 files changed, 17 insertions(+), 36 deletions(-)

--- a/arch/x86/include/asm/sigcontext.h
+++ b/arch/x86/include/asm/sigcontext.h
@@ -57,9 +57,9 @@ struct sigcontext {
unsigned long ip;
unsigned long flags;
unsigned short cs;
- unsigned short __pad2; /* Was called gs, but was always zero. */
- unsigned short __pad1; /* Was called fs, but was always zero. */
- unsigned short ss;
+ unsigned short gs;
+ unsigned short fs;
+ unsigned short __pad0;
unsigned long err;
unsigned long trapno;
unsigned long oldmask;
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -177,24 +177,9 @@ struct sigcontext {
__u64 rip;
__u64 eflags; /* RFLAGS */
__u16 cs;
-
- /*
- * Prior to 2.5.64 ("[PATCH] x86-64 updates for 2.5.64-bk3"),
- * Linux saved and restored fs and gs in these slots. This
- * was counterproductive, as fsbase and gsbase were never
- * saved, so arch_prctl was presumably unreliable.
- *
- * If these slots are ever needed for any other purpose, there
- * is some risk that very old 64-bit binaries could get
- * confused. I doubt that many such binaries still work,
- * though, since the same patch in 2.5.64 also removed the
- * 64-bit set_thread_area syscall, so it appears that there is
- * no TLS API that works in both pre- and post-2.5.64 kernels.
- */
- __u16 __pad2; /* Was gs. */
- __u16 __pad1; /* Was fs. */
-
- __u16 ss;
+ __u16 gs;
+ __u16 fs;
+ __u16 __pad0;
__u64 err;
__u64 trapno;
__u64 oldmask;
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -93,8 +93,15 @@ int restore_sigcontext(struct pt_regs *r
COPY(r15);
#endif /* CONFIG_X86_64 */

+#ifdef CONFIG_X86_32
COPY_SEG_CPL3(cs);
COPY_SEG_CPL3(ss);
+#else /* !CONFIG_X86_32 */
+ /* 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. */
+ COPY_SEG_CPL3(cs);
+#endif /* CONFIG_X86_32 */

get_user_ex(tmpflags, &sc->flags);
regs->flags = (regs->flags & ~FIX_EFLAGS) | (tmpflags & FIX_EFLAGS);
@@ -154,9 +161,8 @@ int setup_sigcontext(struct sigcontext _
#else /* !CONFIG_X86_32 */
put_user_ex(regs->flags, &sc->flags);
put_user_ex(regs->cs, &sc->cs);
- put_user_ex(0, &sc->__pad2);
- put_user_ex(0, &sc->__pad1);
- put_user_ex(regs->ss, &sc->ss);
+ put_user_ex(0, &sc->gs);
+ put_user_ex(0, &sc->fs);
#endif /* CONFIG_X86_32 */

put_user_ex(fpstate, &sc->fpstate);
@@ -450,19 +456,9 @@ static int __setup_rt_frame(int sig, str

regs->sp = (unsigned long)frame;

- /*
- * Set up the CS and SS registers to run signal handlers in
- * 64-bit mode, even if the handler happens to be interrupting
- * 32-bit or 16-bit code.
- *
- * SS is subtle. In 64-bit mode, we don't need any particular
- * SS descriptor, but we do need SS to be valid. It's possible
- * that the old SS is entirely bogus -- this can happen if the
- * signal we're trying to deliver is #GP or #SS caused by a bad
- * SS value.
- */
+ /* 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;
- regs->ss = __USER_DS;

return 0;
}