x86/signal/64: Remove 'fs' and 'gs' from sigcontext

As far as I can tell, these fields have been set to zero on save
and ignored on restore since Linux was imported into git.
Rename them '__pad1' and '__pad2' to avoid confusion.  This may
also allow us to recycle them some day.

This also adds a comment clarifying the history of those fields.

I'm intentionally avoiding calling either of them '__pad0': the
field formerly known as '__pad0' is now 'ss'.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/844f8490e938780c03355be4c9b69eb4c494bf4e.1426193719.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
index f910cdcb71fde99640c8c6d0d3fd254d82f77be1..6fe6b182c9981dd891a9a5bc9a55b3e6591a6f9f 100644 (file)
--- a/arch/x86/include/asm/sigcontext.h
+++ b/arch/x86/include/asm/sigcontext.h
@@ -57,8 +57,8 @@ struct sigcontext {
        unsigned long ip;
        unsigned long flags;
        unsigned short cs;
-       unsigned short gs;
-       unsigned short fs;
+       unsigned short __pad2;  /* Was called gs, but was always zero. */
+       unsigned short __pad1;  /* Was called fs, but was always zero. */
        unsigned short ss;
        unsigned long err;
        unsigned long trapno;

diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index 076b11fd6fa1e9fff00ba6d46e761f1545a6fe03..16dc4e8a2cd34845042445915f9e9d74d90546c6 100644 (file)
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -177,8 +177,23 @@ struct sigcontext {
        __u64 rip;
        __u64 eflags;           /* RFLAGS */
        __u16 cs;
-       __u16 gs;
-       __u16 fs;
+
+       /*
+        * 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;
        __u64 err;
        __u64 trapno;

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index e2f6061a9003a88b917ebd3ee02880b3b5764993..edcb862cdcae5a5b62cbed4b374bb133248a934a 100644 (file)
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -155,8 +155,8 @@ 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->gs);
-               put_user_ex(0, &sc->fs);
+               put_user_ex(0, &sc->__pad2);
+               put_user_ex(0, &sc->__pad1);
                put_user_ex(regs->ss, &sc->ss);
 #endif /* CONFIG_X86_32 */