From f1f029c7bfbf4ee1918b90a431ab823bed812504 Mon Sep 17 00:00:00 2001 From: "H. Peter Anvin" Date: Mon, 3 Aug 2009 16:33:40 -0700 Subject: [PATCH] x86: fix assembly constraints in native_save_fl() From Gabe Black in bugzilla 13888: native_save_fl is implemented as follows: 11static inline unsigned long native_save_fl(void) 12{ 13 unsigned long flags; 14 15 asm volatile("# __raw_save_flags\n\t" 16 "pushf ; pop %0" 17 : "=g" (flags) 18 : /* no input */ 19 : "memory"); 20 21 return flags; 22} If gcc chooses to put flags on the stack, for instance because this is inlined into a larger function with more register pressure, the offset of the flags variable from the stack pointer will change when the pushf is performed. gcc doesn't attempt to understand that fact, and address used for pop will still be the same. It will write to somewhere near flags on the stack but not actually into it and overwrite some other value. I saw this happen in the ide_device_add_all function when running in a simulator I work on. I'm assuming that some quirk of how the simulated hardware is set up caused the code path this is on to be executed when it normally wouldn't. A simple fix might be to change "=g" to "=r". Reported-by: Gabe Black Signed-off-by: H. Peter Anvin Cc: Stable Team --- arch/x86/include/asm/irqflags.h | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h index 2bdab21..c6ccbe7 100644 --- a/arch/x86/include/asm/irqflags.h +++ b/arch/x86/include/asm/irqflags.h @@ -12,9 +12,15 @@ static inline unsigned long native_save_fl(void) { unsigned long flags; + /* + * Note: this needs to be "=r" not "=rm", because we have the + * stack offset from what gcc expects at the time the "pop" is + * executed, and so a memory reference with respect to the stack + * would end up using the wrong address. + */ asm volatile("# __raw_save_flags\n\t" "pushf ; pop %0" - : "=g" (flags) + : "=r" (flags) : /* no input */ : "memory"); -- 1.8.2.3