| Message ID | 20180312155524.b421f07d7f08f24c57bd1887@linux-foundation.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton <akpm@linux-foundation.org> wrote: > > Replacing the __builtin_choose_expr() with ?: works of course. Hmm. That sounds like the right thing to do. We were so myopically staring at the __builtin_choose_expr() problem that we overlooked the obvious solution. Using __builtin_constant_p() together with a ?: is in fact our common pattern, so that should be fine. The only real reason to use __builtin_choose_expr() is if you want to get the *type* to vary depending on which side you choose, but that's not an issue for min/max. > What will be the runtime effects? There should be none. Gcc will turn the conditional for the ?: into a constant, and DTRT. Linus
On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton > <akpm@linux-foundation.org> wrote: >> >> Replacing the __builtin_choose_expr() with ?: works of course. > > Hmm. That sounds like the right thing to do. We were so myopically > staring at the __builtin_choose_expr() problem that we overlooked the > obvious solution. > > Using __builtin_constant_p() together with a ?: is in fact our common > pattern, so that should be fine. The only real reason to use > __builtin_choose_expr() is if you want to get the *type* to vary > depending on which side you choose, but that's not an issue for > min/max. This doesn't solve it for -Wvla, unfortunately. That was the point of Josh's original suggestion of __builtin_choose_expr(). Try building with KCFLAGS=-Wval and checking net/ipv6/proc.c: net/ipv6/proc.c: In function ‘snmp6_seq_show_item’: net/ipv6/proc.c:198:2: warning: ISO C90 forbids array ‘buff’ whose size can’t be evaluated [-Wvla] unsigned long buff[SNMP_MIB_MAX]; ^~~~~~~~ -Kees
On Mon, 12 Mar 2018 21:28:57 -0700 Kees Cook <keescook@chromium.org> wrote: > On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton > > <akpm@linux-foundation.org> wrote: > >> > >> Replacing the __builtin_choose_expr() with ?: works of course. > > > > Hmm. That sounds like the right thing to do. We were so myopically > > staring at the __builtin_choose_expr() problem that we overlooked the > > obvious solution. > > > > Using __builtin_constant_p() together with a ?: is in fact our common > > pattern, so that should be fine. The only real reason to use > > __builtin_choose_expr() is if you want to get the *type* to vary > > depending on which side you choose, but that's not an issue for > > min/max. > > This doesn't solve it for -Wvla, unfortunately. That was the point of > Josh's original suggestion of __builtin_choose_expr(). > > Try building with KCFLAGS=-Wval and checking net/ipv6/proc.c: > > net/ipv6/proc.c: In function ‘snmp6_seq_show_item’: > net/ipv6/proc.c:198:2: warning: ISO C90 forbids array ‘buff’ whose > size can’t be evaluated [-Wvla] > unsigned long buff[SNMP_MIB_MAX]; > ^~~~~~~~ PITA. Didn't we once have a different way of detecting VLAs? Some post-compilation asm parser, iirc. I suppose the world wouldn't end if we had a gcc version ifdef in kernel.h. We'll get to remove it in, oh, ten years.
On Tue, Mar 13, 2018 at 2:02 PM, Andrew Morton <akpm@linux-foundation.org> wrote: > On Mon, 12 Mar 2018 21:28:57 -0700 Kees Cook <keescook@chromium.org> wrote: > >> On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds >> <torvalds@linux-foundation.org> wrote: >> > On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton >> > <akpm@linux-foundation.org> wrote: >> >> >> >> Replacing the __builtin_choose_expr() with ?: works of course. >> > >> > Hmm. That sounds like the right thing to do. We were so myopically >> > staring at the __builtin_choose_expr() problem that we overlooked the >> > obvious solution. >> > >> > Using __builtin_constant_p() together with a ?: is in fact our common >> > pattern, so that should be fine. The only real reason to use >> > __builtin_choose_expr() is if you want to get the *type* to vary >> > depending on which side you choose, but that's not an issue for >> > min/max. >> >> This doesn't solve it for -Wvla, unfortunately. That was the point of >> Josh's original suggestion of __builtin_choose_expr(). >> >> Try building with KCFLAGS=-Wval and checking net/ipv6/proc.c: >> >> net/ipv6/proc.c: In function ‘snmp6_seq_show_item’: >> net/ipv6/proc.c:198:2: warning: ISO C90 forbids array ‘buff’ whose >> size can’t be evaluated [-Wvla] >> unsigned long buff[SNMP_MIB_MAX]; >> ^~~~~~~~ > > PITA. Didn't we once have a different way of detecting VLAs? Some > post-compilation asm parser, iirc. > > I suppose the world wouldn't end if we had a gcc version ifdef in > kernel.h. We'll get to remove it in, oh, ten years. For fixing only 6 VLAs, we don't need all this effort. When it looked like we could get away with just a "better" max(), sure. ;) I'll send a "const_max()" which will refuse to work on non-constant-values (so it doesn't get accidentally used on variables that could be exposed to double-evaluation), and will work for stack array declarations (to avoid the overly-sensitive -Wvla checks). -Kees
From: Kees Cook > Sent: 13 March 2018 22:15 ... > I'll send a "const_max()" which will refuse to work on > non-constant-values (so it doesn't get accidentally used on variables > that could be exposed to double-evaluation), and will work for stack > array declarations (to avoid the overly-sensitive -Wvla checks). ISTR the definitions were of the form: char foo[max(sizeof (struct bah), sizeof (struct baz))]; This doesn't generate a 'foo' with the required alignment. It would be much better to use a union. David
--- a/include/linux/kernel.h~kernelh-skip-single-eval-logic-on-literals-in-min-max-v3-fix +++ a/include/linux/kernel.h @@ -804,13 +804,10 @@ static inline void ftrace_dump(enum ftra * accidental VLA. */ #define __min(t1, t2, x, y) \ - __builtin_choose_expr(__builtin_constant_p(x) && \ - __builtin_constant_p(y), \ - (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y), \ - __single_eval_min(t1, t2, \ - __UNIQUE_ID(min1_), \ - __UNIQUE_ID(min2_), \ - x, y)) + ((__builtin_constant_p(x) && __builtin_constant_p(y)) ? \ + ((t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y)) : \ + (__single_eval_min(t1, t2, __UNIQUE_ID(min1_), \ + __UNIQUE_ID(min2_), x, y))) /** * min - return minimum of two values of the same or compatible types @@ -826,13 +823,11 @@ static inline void ftrace_dump(enum ftra max1 > max2 ? max1 : max2; }) #define __max(t1, t2, x, y) \ - __builtin_choose_expr(__builtin_constant_p(x) && \ - __builtin_constant_p(y), \ - (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y), \ - __single_eval_max(t1, t2, \ - __UNIQUE_ID(max1_), \ - __UNIQUE_ID(max2_), \ - x, y)) + ((__builtin_constant_p(x) && __builtin_constant_p(y)) ? \ + ((t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y)) : \ + (__single_eval_max(t1, t2, __UNIQUE_ID(max1_), \ + __UNIQUE_ID(max2_), x, y))) + /** * max - return maximum of two values of the same or compatible types * @x: first value