On Tue, Mar 1, 2022 at 4:46 PM Ammar Faizi wrote: > Fortunately, the constraint violation that's fixed by patch 1 doesn't > yield any bug due to the nature of System V ABI. Should we backport > this? hi sir, it might also be interesting to know that even if it never be inlined, it's still potential to break. for example this code (https://godbolt.org/z/xWMTxhTET) __attribute__((__noinline__)) static void x(int a) { asm("xorl\t%%r8d, %%r8d"::"a"(a)); } extern int p(void); int f(void) { int ret = p(); x(ret); return ret; } translates to this asm x: movl %edi, %eax xorl %r8d, %r8d ret f: subq $8, %rsp call p movl %eax, %r8d movl %eax, %edi call x movl %r8d, %eax addq $8, %rsp ret See the %r8d? It should be clobbered by a function call too. But since no one tells the compiler that we clobber %r8d, it assumes %r8d never changes after that call. The compiler thinks x() is static and will not clobber %r8d, even the ABI says %r8d will be clobbered by a function call. So i think it should be backported to the stable kernel, it's still a fix -- Viro
