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Message-ID: <ZHn6g+9RbQs+kuHz@voyager>
Date: Fri, 2 Jun 2023 16:19:47 +0200
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] [RFC] make clone() usable
Am Thu, Jun 01, 2023 at 04:08:07PM -0400 schrieb Rich Felker:
> On Thu, Jun 01, 2023 at 01:12:57PM +0300, Alexey Izbyshev wrote:
> > Another thing that might be somewhat relevant if we expect to see
> > wider usage of clone is that syscall (the public function) currently
> > blindly extracts six arguments via va_arg, which on some
> > architectures requires sufficient stack space not to crash. So on
> > i386 the following silly function will crash if passed to clone,
> > provided that "stack" points to the beginning of unmapped space past
> > the stack page end.
>
> That's changed in the patch. It only calls va_arg for arguments whose
> existence the flags implies.
>
> Rich
clone() does, yes, but syscall() doesn't. So if the function handed to
clone() calls the syscall() function at the top of the stack, it crashes
on architectures that pass parameters on stack primarily.
Quick fix would be to make __clone() on those architectures reserve
sufficient stack space for six parameters before calling the client
function. E.g. on i386, one parameter is four bytes, so six parameters
are twenty-four bytes, and I believe the stack alignment was sixteen
bytes, right? So __clone() could just reduce %esp by thirty-two before
calling the function pointer.
The proper fix would be to have syscall() only read as many arguments as
were given. Since syscall() is an extension function, nobody can rely on
it being defined as
long syscall(int, ...);
You might use similar macros to the ones in syscall.h to add another
argument for the number of arguments. That would avoid the crash.
Of course, you have to keep the above function around for ABI reasons.
Nothing's ever easy, right?
Ciao,
Markus
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