[PATCH] x86/lib: make clean_cache_range() zero-size safe

["Li Zhe" <lizhe.67@bytedance.com>]

clean_cache_range() writes back each cache line in the range
[addr, addr + size). A zero-size range is empty and should not perform
any cache maintenance operation.

As pointed out by Sashiko [1], clean_cache_range(addr, 0) currently can
still execute one CLWB when addr is not cache-line aligned. With size 0,
vend is equal to addr. However, the loop starts from the
cacheline-aligned address containing addr. If addr is not cacheline
aligned, that rounded-down start is below vend, so the loop can execute
one CLWB even though the requested range is empty.

That gives zero-size callers observable side effects. For example,
arch_wb_cache_pmem(addr, 0) should not write back any cache line, and
memcpy_flushcache(dst, src, 0) should preserve the usual zero-length
copy semantics. If the rounded-down line is not mapped, the stray CLWB
can also fault.

Return immediately from clean_cache_range() for size 0.

[1] https://sashiko.dev/#/patchset/20260831111638.76012-1-lizhe.67@bytedance.com

Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
 arch/x86/lib/usercopy_64.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index c47d8cd0e243..5adf772cbf04 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -32,6 +32,9 @@ static void clean_cache_range(void *addr, size_t size)
 	void *vend = addr + size;
 	void *p;
 
+	if (!size)
+		return;
+
 	for (p = (void *)((unsigned long)addr & ~clflush_mask);
 	     p < vend; p += x86_clflush_size)
 		clwb(p);
-- 
2.20.1