On Thu, Nov 04, 2021 at 02:35:40PM +0100, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@xxxxxxxx> > > gcc warns about potentially undefined behavior in an array index: > > mm/vmalloc.c: In function 'vmap_pfn_apply': > mm/vmalloc.c:2800:58: error: operation on 'data->idx' may be undefined [-Werror=sequence-point] > 2800 | *pte = pte_mkspecial(pfn_pte(data->pfns[data->idx++], data->prot)); > | ~~~~~~~~~^~ > arch/arm64/include/asm/pgtable-types.h:25:37: note: in definition of macro '__pte' > 25 | #define __pte(x) ((pte_t) { (x) } ) > | ^ > arch/arm64/include/asm/pgtable.h:80:15: note: in expansion of macro '__phys_to_pte_val' > 80 | __pte(__phys_to_pte_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot)) > | ^~~~~~~~~~~~~~~~~ > mm/vmalloc.c:2800:30: note: in expansion of macro 'pfn_pte' > 2800 | *pte = pte_mkspecial(pfn_pte(data->pfns[data->idx++], data->prot)); > | ^~~~~~~ > > This only appeared in one randconfig build so far, and I don't know > what caused it, but moving the index increment out of the expression > at least addresses the warning. Would that randconfig include CONFIG_ARM64_PA_BITS_52? #define __phys_to_pte_val(phys) (((phys) | ((phys) >> 36)) & PTE_ADDR_MASK) because that's going to double-increment idx. Or single increment. Or whatever else the compiler feels like doing.