Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

From: Linus Torvalds
Date: Thu Aug 13 2015 - 12:19:26 EST

On Thu, Aug 13, 2015 at 8:43 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> I'm trying to fix it without reverting. If that doesn't work, then we
> revert. Yesterday, I thought I had a reasonably clean fix, but it
> turned out that it only solved half of the problem.

The thing is, I actually think that the current situation is crazy.

Especially given that we don't restore any of the other segment
registers on x86-64 (except CS, of course)

So how about this "alternate" minimal patch instead. The difference is:

- we actually leave the

regs->ss = __USER_DS;

in __setup_rt_frame, to guarantee that when we take a signal, we do
take it with a valid SS

- but it removes all the other games with SS (and treats it exactly
the same as FS/GS).

So now we don't play games with the actual sigcontext, and
hopefully dosemu is happier.

Hmm? That actually makes the code look better, and doesn't
re-introduce that annoying CONFIG_X86_32 case (because it now does it
in obviously the same place as fs/gs).

So the code is cleaner, and closer to what we used to do.

Stas, can you test this one too? I, like Luto, don't actually have a
dosemu test-case.

arch/x86/include/asm/sigcontext.h | 2 +-
arch/x86/include/uapi/asm/sigcontext.h | 3 +--
arch/x86/kernel/signal.c | 4 ++--
3 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
index 6fe6b182c998..2cefce9b52bd 100644
--- a/arch/x86/include/asm/sigcontext.h
+++ b/arch/x86/include/asm/sigcontext.h
@@ -59,7 +59,7 @@ struct sigcontext {
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 __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..d99bbc2d4b0b 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -193,8 +193,7 @@ struct sigcontext {
__u16 __pad2; /* Was gs. */
__u16 __pad1; /* Was fs. */
- __u16 ss;
+ __u16 __pad0; /* Not ss. */
__u64 err;
__u64 trapno;
__u64 oldmask;
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 206996c1669d..e955f0cd9b9d 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -77,6 +77,7 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
+ COPY_SEG_CPL3(ss);
#endif /* CONFIG_X86_32 */

COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
@@ -94,7 +95,6 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
#endif /* CONFIG_X86_64 */

- COPY_SEG_CPL3(ss);

get_user_ex(tmpflags, &sc->flags);
regs->flags = (regs->flags & ~FIX_EFLAGS) | (tmpflags & FIX_EFLAGS);
@@ -156,7 +156,7 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
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->__pad0);
#endif /* CONFIG_X86_32 */

put_user_ex(fpstate, &sc->fpstate);