linux-kernel - Re: [PATCH v2 10/11] tools/nolibc: add ptrace support

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Message-ID: <20250921081947.GC16684@1wt.eu>
Date: Sun, 21 Sep 2025 10:19:47 +0200
From: Willy Tarreau <w@....eu>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Benjamin Berg <benjamin@...solutions.net>, linux-um@...ts.infradead.org,
        linux-kselftest@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-kernel@...r.kernel.org, Benjamin Berg <benjamin.berg@...el.com>
Subject: Re: [PATCH v2 10/11] tools/nolibc: add ptrace support

On Sat, Sep 20, 2025 at 11:27:19AM +0200, Thomas Weißschuh wrote:
> > +/*
> > + * long ptrace(int op, pid_t pid, void *addr, void *data);
> > + *
> > + * However, addr may also be an integer in some cases.
> 
> This comment is a bit confusing. It reads like it can be an integer as
> in 'int' datatype. But the kernel expects an integer compatible with
> 'void *', so 'unsigned long'. I think we can drop the comment.

Or if the point is to insist that void* is not strictly required, a
comment might as well say "types for addr and data are ignored and
will be cast to void*". But given that the man page indicates two
void*, I think that callers are expected to cast ints to void* in
any case, and if so maybe indeed not saying anything about it would
be better.

> > + */
> > +static __attribute__((unused))
> > +long sys_vptrace(int op, pid_t pid, va_list args)
> > +{
> > +	return my_syscall4(__NR_ptrace, op, pid,
> > +			   va_arg(args, void *), va_arg(args, void *));
> > +}
> > +
> > +static __attribute__((unused))
> > +ssize_t sys_ptrace(int op, pid_t pid, ...)
> 
> ptrace(2) does not document addr and data to be optional.
> While it does acknowledge the fact that it is variadic on glibc,
> users are still recommended to always supply all arguments.
> I'd prefer to keep it simple and avoid the va_list.

The man indeed says they are *ignored*, not optional. I agree then that
it would be better not doing simpler than the standard and have to roll
back later. Let's just pass unused arguments to the system.

Thanks,
Willy