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Subject-Warray-bounds fun again
Date
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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