> 2023年7月19日 16:22，Dan Carpenter <dan.carpenter@xxxxxxxxxx> 写道：
>
> On Wed, Jul 19, 2023 at 03:58:59PM +0800, Menglong Dong wrote:
>>> 1957 /* copy function arguments from origin stack frame
>>> 1958 * into current stack frame.
>>> 1959 *
>>> 1960 * The starting address of the arguments on-stack
>>> 1961 * is:
>>> 1962 * rbp + 8(push rbp) +
>>> 1963 * 8(return addr of origin call) +
>>> 1964 * 8(return addr of the caller)
>>> 1965 * which means: rbp + 24
>>> 1966 */
>>> 1967 for (j = 0; j < arg_regs; j++) {
>>> 1968 emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
>>> 1969 nr_stack_slots * 8 + 0x18);
>>> 1970 emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
>>> 1971 -stack_size);
>>> 1972
>>> 1973 if (!nr_stack_slots)
>>> 1974 first_off = stack_size;
>>> 1975 stack_size -= 8;
>>> 1976 nr_stack_slots++;
>>> 1977 }
>>> 1978 } else {
>>> 1979 /* Only copy the arguments on-stack to current
>>> 1980 * 'stack_size' and ignore the regs, used to
>>> 1981 * prepare the arguments on-stack for orign call.
>>> 1982 */
>>> 1983 if (for_call_origin) {
>>> 1984 nr_regs += arg_regs;
>>> 1985 continue;
>>> 1986 }
>>> 1987
>>> 1988 /* copy the arguments from regs into stack */
>>> 1989 for (j = 0; j < arg_regs; j++) {
>>> 1990 emit_stx(prog, BPF_DW, BPF_REG_FP,
>>> 1991 nr_regs == 5 ? X86_REG_R9 : BPF_REG_1 + nr_regs,
>>> 1992 -stack_size);
>>> 1993 stack_size -= 8;
>>> 1994 nr_regs++;
>>> 1995 }
>>> 1996 }
>>> 1997 }
>>> 1998
>>> --> 1999 clean_stack_garbage(m, prog, nr_stack_slots, first_off);
>>> 2000 }
>>
>> Hello,
>>
>> Thanks for the reporting. The variable ‘first_off’ that passed to
>> clean_stack_garbage() should be ok, as it is only used when
>> "nr_stack_slots == 1”, in which case “first_off” should already be
>> initialized.
>>
>> (Anyway, maybe we should initialize it to avoid passing a
>> uninitialized variable to a function?)
>
> There is no downside to initializing it to zero. Everyone does that
> automatically these days.
>
> In a case where the variable is not used, it depends on if the function
> is inlined or not. If it's inlined, then it's not a bug. If it isn't
> inlined then it's still a bug whether the variable is used or not.
> It's undefined behavior in C and it can cause runtime warnings with
> KMEMsan.
Yeah, that make sense. I’ll send a patch to fix it.
Thanks!
Menglong Dong
>
> regards,
> dan carpenter
>
