Re: [PATCH bpf-next v5 2/3] bpf, x86: allow function arguments up to 12 for TRACING

[Menglong Dong]

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
>
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