Revert x86 sigcontext cleanups

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>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 17 insertions, 36 deletions

diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
index 6fe6b182c998..9dfce4e0417d 100644
--- 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;
diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index 0e8a973de9ee..40836a9a7250 100644
--- 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;
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 206996c1669d..71820c42b6ce 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -93,8 +93,15 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
                 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 __user *sc, void __user *fpstate,
 #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);
@@ -451,19 +457,9 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
 
         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;
 }