From 0fcedc8631ec28ca25d3c0b116e8fa0c19dd5f6d Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Fri, 16 Jan 2015 16:19:27 -0800 Subject: [PATCH] x86_64 entry: Fix RCX for ptraced syscalls The int_ret_from_sys_call and syscall tracing code disagrees with the sysret path as to the value of RCX. The Intel SDM, the AMD APM, and my laptop all agree that sysret returns with RCX == RIP. The syscall tracing code does not respect this property. For example, this program: int main() { extern const char syscall_rip[]; unsigned long rcx = 1; unsigned long orig_rcx = rcx; asm ("mov $-1, %%eax\n\t" "syscall\n\t" "syscall_rip:" : "+c" (rcx) : : "r11"); printf("syscall: RCX = %lX RIP = %lX orig RCX = %lx\n", rcx, (unsigned long)syscall_rip, orig_rcx); return 0; } prints: syscall: RCX = 400556 RIP = 400556 orig RCX = 1 Running it under strace gives this instead: syscall: RCX = FFFFFFFFFFFFFFFF RIP = 400556 orig RCX = 1 This changes FIXUP_TOP_OF_STACK to match sysret, causing the test to show RCX == RIP even under strace. It looks like this is a partial revert of: 88e4bc32686e ("[PATCH] x86-64 architecture specific sync for 2.5.8") from the historic git tree. Signed-off-by: Andy Lutomirski Cc: Frederic Weisbecker Cc: Oleg Nesterov Cc: Borislav Petkov Cc: Rik van Riel Cc: Linus Torvalds Link: http://lkml.kernel.org/r/c9a418c3dc3993cb88bb7773800225fd318a4c67.1421453410.git.luto@amacapital.net Signed-off-by: Ingo Molnar --- arch/x86/kernel/entry_64.S | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 9ebaf63ba182..c653dc437e6b 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -143,7 +143,8 @@ ENDPROC(native_usergs_sysret64) movq \tmp,RSP+\offset(%rsp) movq $__USER_DS,SS+\offset(%rsp) movq $__USER_CS,CS+\offset(%rsp) - movq $-1,RCX+\offset(%rsp) + movq RIP+\offset(%rsp),\tmp /* get rip */ + movq \tmp,RCX+\offset(%rsp) /* copy it to rcx as sysret would do */ movq R11+\offset(%rsp),\tmp /* get eflags */ movq \tmp,EFLAGS+\offset(%rsp) .endm -- 2.45.2