Messages in this thread | | | Date | Mon, 23 Apr 2018 14:50:13 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/entry/64/compat: Preserve r8-r11 in int $0x80 |
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On Tue, Apr 17, 2018 at 07:36:36AM -0700, Andy Lutomirski wrote: > 32-bit user code that uses int $80 doesn't care about r8-r11. There is, > however, some 64-bit user code that intentionally uses int $0x80 to > invoke 32-bit system calls. From what I've seen, basically all such > code assumes that r8-r15 are all preserved, but the kernel clobbers > r8-r11. Since I doubt that there's any code that depends on int $0x80 > zeroing r8-r11, change the kernel to preserve them. > > I suspect that very little user code is broken by the old clobber, > since r8-r11 are only rarely allocated by gcc, and they're clobbered > by function calls, so they only way we'd see a problem is if the > same function that invokes int $0x80 also spills something important > to one of these registers. > > The current behavior seems to date back to the historical commit > "[PATCH] x86-64 merge for 2.6.4". Before that, all regs were > preserved. I can't find any explanation of why this change was made.
Probably because r8-r11 are callee-clobbered, according to ABI so someone decided to whack them so that code which doesn't adhere to the ABI would fall on its face...
Also, looking at PUSH_AND_CLEAR_REGS and how we call it on the 64-bit entry path, we probably should keep clearing those regs to avoid speculation crap.
Methinks.
-- Regards/Gruss, Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
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