Messages in this thread | | | From | Sven Schnelle <> | Subject | -Warray-bounds fun again | Date | Thu, 21 Apr 2022 16:02:07 +0200 |
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Hi,
while compiling the latest upstream kernel on fedora 36 which uses gcc-12 by default, i got a lot of -Warray-bounds warnings:
(Note that this is on s390 arch)
In function ‘preempt_count’, inlined from ‘do_one_initcall’ at init/main.c:1290:14: ./include/asm-generic/rwonce.h:44:26: warning: array subscript 0 is outside array bounds of ‘const volatile int[0]’ [-Warray-bounds] 44 | #define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x)) | ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /include/asm-generic/rwonce.h:50:9: note: in expansion of macro ‘__READ_ONCE’ 50 | __READ_ONCE(x); | ^~~~~~~~~~~ ./arch/s390/include/asm/preempt.h:17:16: note: in expansion of macro ‘READ_ONCE’ 17 | return READ_ONCE(S390_lowcore.preempt_count) & ~PREEMPT_NEED_RESCHED; | ^~~~~~~~~
This is because S390_lowcore is defined as follows:
#define S390_lowcore (*((struct lowcore *) 0))
Lowcore is a 8K cpu-local memory region on s390 at fixed address 0.
The obvious 'fix' is to use absolute_pointer():
#define S390_lowcore (*((struct lowcore *)absolute_pointer(0)))
That makes the warning go away, but unfortunately the compiler no longer knows that the memory access is fitting into a load/store with a 12 bit displacement.
Without absolute_pointer(), reading the preempt count is just a single instruction: 'l %r11,936'
static inline int preempt_count(void) { return READ_ONCE(S390_lowcore.preempt_count) & ~PREEMPT_NEED_RESCHED; 8c4: 58 b0 03 a8 l %r11,936 <--- load preempt count 8c8: b9 04 00 92 lgr %r9,%r2 int count = preempt_count();
with absolute pointer(), the compiler no longer optimizes the read to one instruction and uses an additional base register:
static inline int preempt_count(void) { return READ_ONCE(S390_lowcore.preempt_count) & ~PREEMPT_NEED_RESCHED; 8c4: a7 19 00 00 lghi %r1,0 <-- use %r1 as base, load with 0 8c8: b9 04 00 92 lgr %r9,%r2 int count = preempt_count(); char msgbuf[64]; 8cc: d7 3f f0 a8 f0 a8 xc 168(64,%r15),168(%r15) 8d2: 58 b0 13 a8 l %r11,936(%r1) <-- and finally add the offset and fetch int ret;
The reason for gcc to not optimize that further is likely the asm statement in RELOC_HIDE (located in include/linux/compiler-gcc.h)
#define RELOC_HIDE(ptr, off) \ ({ \ unsigned long __ptr; \ __asm__ ("" : "=r"(__ptr) : "0"(ptr)); \ (typeof(ptr)) (__ptr + (off)); \ })
For most of the code this wouldn't be a big problem, but we're storing information like preempt_count, current thread info, etc in lowcore because it is the fastest way. I would like to avoid to use additional instructions/registers just to avoid a warning.
Does anyone have an idea about a different way to make this warning go away?
Thanks Sven
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