On Thu, Jun 22, 2023 at 5:06 PM David Laight <David.Laight@xxxxxxxxxx> wrote: > > ... > > > + /* Generally speaking, the compiler will pass the arguments > > > + * on-stack with "push" instruction, which will take 8-byte > > > + * on the stack. On this case, there won't be garbage values > > > > On this case -> In this case. The same for below another case. > > > > > + * while we copy the arguments from origin stack frame to current > > > + * in BPF_DW. > > > + * > > > + * However, sometimes the compiler will only allocate 4-byte on > > > + * the stack for the arguments. For now, this case will only > > > + * happen if there is only one argument on-stack and its size > > > + * not more than 4 byte. On this case, there will be garbage > > > + * values on the upper 4-byte where we store the argument on > > > + * current stack frame. > > Is that right for 86-64? > > IIRC arguments always take (at least) 64bits. > For any 32bit argument (register or stack) the high bits are undefined. > (Maybe in kernel they are always zero? > From 32bit userspace they are definitely random.) > Hello, According to my testing, the compiler will always pass the arguments on 8-byte size with "push" insn if the count of the arguments that need to be passed on stack more than 1 and the size of the argument doesn't exceed 8-byte. In this case, there won't be garbage. For example, the high 4-byte will be made 0 if the size of the argument is 4-byte, as the "push" insn will copy the argument from regs or imm into stack in 8-byte. If the count of the arguments on-stack is 1 and its size doesn't exceed 4-byte, some compiler, like clang, may not use the "push" insn. Instead, it allocates 4 bytes in the stack, and copies the arguments from regs or imm into stack in 4-byte. This is the case we deal with here. I'm not sure if I understand you correctly. Do you mean that there will be garbage values for 32bit args? > I think the called code is also responsible form masking 8 and 16bit > values (in reality char/short args and return values just add code > bloat). > > A 128bit value is either passed in two registers or two stack > slots. If the last register is skipped it will be used for the > next argument. > Yeah, this point is considered in save_args(). Once this happen, the count of stack slots should more then 1, and the arguments on-stack will be stored with "push" insn in 8-byte. Therefore, there shouldn't be garbage values in this case? Do I miss something? Thanks! Menglong Dong > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales)
