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

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Date:   Thu, 22 Jun 2023 09:06:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Yonghong Song' <yhs@...a.com>,
        "menglong8.dong@...il.com" <menglong8.dong@...il.com>,
        "alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "martin.lau@...ux.dev" <martin.lau@...ux.dev>,
        "song@...nel.org" <song@...nel.org>, "yhs@...com" <yhs@...com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kpsingh@...nel.org" <kpsingh@...nel.org>,
        "sdf@...gle.com" <sdf@...gle.com>,
        "haoluo@...gle.com" <haoluo@...gle.com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "benbjiang@...cent.com" <benbjiang@...cent.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Menglong Dong <imagedong@...cent.com>
Subject: RE: [PATCH bpf-next v5 2/3] bpf, x86: allow function arguments up to
 12 for TRACING

...
> > +	/* 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.)

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.

	David

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