Messages in this thread | | | Date | Tue, 7 Feb 2017 16:03:01 +0000 | From | Mark Rutland <> | Subject | Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA_CORRUPTION |
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On Tue, Feb 07, 2017 at 04:07:37PM +0100, Peter Zijlstra wrote: > On Tue, Feb 07, 2017 at 01:50:20PM +0000, Mark Rutland wrote: > > > Right something along those lines. (a) will need GCC help, and (b) would > > > be kernel-arch specific. So this isn't something we can quickly do. > > > > I agree this isn't something that can be hacked together quickly, and > > certainly shouldn't block these patches. > > > > However, I don't think we need anything new from GCC, and I think we > > already have a generic API for (b). > > > > For (a) we don't need new GCC help if we do something like we did in > > commit 72c5839515260dce to do the mangling. Prepend a prefix to the > > register, e.g. changing 'x0' to '__pt_regs_offset_x0', which we arrange > > to hold the correct value. > > I'm not sure I can decipher that commit and therefore have no idea if > something similar can be done for other architectures.
For x86 it's a little painful due to '%' in the register names, but it looks possible. The below appears to do the mangling correctly (then screams due to the mangled result being nonexistent).
Thanks, Mark.
---->8---- #define cmpxchg(ptr, old, new) \ ({ \ typeof(*ptr) __ret; \ typeof(*ptr) __old = (old); \ typeof(*ptr) __new = (new); \ \ volatile unsigned int *__ptr = (volatile unsigned int *)ptr; \ asm volatile("cmpxchgl %2, %1" \ : "=a" (__ret), "+m" (*__ptr) \ : "r" (__new), "0" (__old) \ : "memory"); \ __ret; \ })
asm( " .macro reg_to_offset r\n" " .irp rs,eax,ebx,ecx,edx\n" " .ifc \\r, %\\rs\n" " __offset_of_\\rs\n" " .endif\n" " .endr\n" " .endm\n" );
#define asm_sym(var) asm volatile("reg_to_offset %0\n" : : "r" (var))
int foo(void) { unsigned int mem = 0; unsigned int new; int bar = 7, baz = 11;
new = cmpxchg(&mem, 1, 2); asm_sym(new); }
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