Messages in this thread | | | Subject | Re: GCC bug ? Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection | From | Christophe Leroy <> | Date | Wed, 22 Jan 2020 07:52:02 +0100 |
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Le 21/01/2020 à 20:55, Segher Boessenkool a écrit : > On Tue, Jan 21, 2020 at 05:22:32PM +0000, Christophe Leroy wrote: >> g1() should return 3, not 5. > > What makes you say that?
What makes me say that is that NULL is obviously a constant pointer and I think we are all expecting gcc to see it as a constant during kernel build, ie at -O2
> > "A return of 0 does not indicate that the > value is _not_ a constant, but merely that GCC cannot prove it is a > constant with the specified value of the '-O' option." > > (And the rules it uses for this are *not* the same as C "constant > expressions" or C "integer constant expression" or C "arithmetic > constant expression" or anything like that -- which should be already > obvious from that it changes with different -Ox). > > You can use builtin_constant_p to have the compiler do something better > if the compiler feels like it, but not anything more. Often people > want stronger guarantees, but when they see how much less often it then > returns "true", they do not want that either. >
in asm/book3s/64/kup-radix.h we have:
static inline void allow_user_access(void __user *to, const void __user *from, unsigned long size) { // This is written so we can resolve to a single case at build time if (__builtin_constant_p(to) && to == NULL) set_kuap(AMR_KUAP_BLOCK_WRITE); else if (__builtin_constant_p(from) && from == NULL) set_kuap(AMR_KUAP_BLOCK_READ); else set_kuap(0); }
and in asm/kup.h we have:
static inline void allow_read_from_user(const void __user *from, unsigned long size) { allow_user_access(NULL, from, size); }
static inline void allow_write_to_user(void __user *to, unsigned long size) { allow_user_access(to, NULL, size); }
If GCC doesn't see NULL as a constant, then the above doesn't work as expected.
What's surprising and frustrating is that if you remove the __builtin_constant_p() and only leave the NULL check, then GCC sees it as a constant and drops the other leg.
So if we remove the __builtin_constant_p(to) and leave only the (to == NULL), it will work as expected for allow_read_from_user(). But for the others where (to) is not a constant, the NULL test will remain together with the associated leg.
Christophe
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