Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
From: Will Deacon <will@kernel.org>
Date: 2019-12-19 12:11:23
Also in:
linux-arch, lkml
On Tue, Dec 17, 2019 at 10:32:35AM -0800, Linus Torvalds wrote:
On Tue, Dec 17, 2019 at 10:04 AM Linus Torvalds [off-list ref] wrote:quoted
Let me think about it.How about we just get rid of the union entirely, and just use 'unsigned long' or 'unsigned long long' depending on the size. Something like the attached patch - it still requires that it be an arithmetic type, but now because of the final cast. But it might still be a cast to a volatile type, of course. Then the result will be volatile, but at least now READ_ONCE() won't be taking the address of a volatile variable on the stack - does that at least fix some of the horrible code generation. Hmm?
Sounds like it according to mpe, but I'll confirm too for arm64.
This is untested, because I obviously still have the cases of
structures (page table entries) being accessed once..
Linusquoted hunk ↗ jump to hunk
include/linux/compiler.h | 33 +++++++++++++++++---------------- 1 file changed, 17 insertions(+), 16 deletions(-)diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 5e88e7e33abe..8b4282194f16 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h@@ -179,18 +179,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, #include <uapi/linux/types.h> -#define __READ_ONCE_SIZE \ -({ \ - switch (size) { \ - case 1: *(__u8 *)res = *(volatile __u8 *)p; break; \ - case 2: *(__u16 *)res = *(volatile __u16 *)p; break; \ - case 4: *(__u32 *)res = *(volatile __u32 *)p; break; \ - case 8: *(__u64 *)res = *(volatile __u64 *)p; break; \ - default: \ - barrier(); \ - __builtin_memcpy((void *)res, (const void *)p, size); \ - barrier(); \ - } \ +/* "unsigned long" or "unsigned long long" - make it fit in a register if possible */ +#define __READ_ONCE_TYPE(size) \ + __typeof__(__builtin_choose_expr(size > sizeof(0UL), 0ULL, 0UL))
Ha, I wondered when '__builtin_choose_expr()' would make an appearance in this thread! Nice trick. I'll try integrating this with what I have and see what I run into next. Back down the rabbit hole... Will