[tip:x86/asm] x86/signal/64: Remove 'fs' and 'gs' from sigcontext

tip-bot for Andy Lutomirski
2015-03-17T08:45:15Z

Commit-ID: 9a036b93a344235b7899401d04e97c34f3a2554c
Gitweb: http://git.kernel.org/tip/9a036b93a344235b7899401d04e97c34f3a2554c
Author: Andy Lutomirski <luto@amacapital.net>
AuthorDate: Thu, 12 Mar 2015 13:57:52 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 17 Mar 2015 09:25:26 +0100

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>
---
arch/x86/include/asm/sigcontext.h | 4 ++--
arch/x86/include/uapi/asm/sigcontext.h | 19 +++++++++++++++++--
arch/x86/kernel/signal.c | 4 ++--
3 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
index f910cdc..6fe6b18 100644
--- 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 076b11f..16dc4e8 100644
--- 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 e2f6061..edcb862 100644
--- 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 */