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Date: Tue, 2 Jan 2018 14:49:09 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Add comments to i386 assembly source
On Mon, Jan 01, 2018 at 07:15:50PM -0800, John Reiser wrote:
> On 01/01/2018 13:49 UTC, Rich Felker wrote:
> >On Mon, Jan 01, 2018 at 02:57:02PM -0800, John Reiser wrote:
> >>There's a bug. clone() is a user-level function that can be used
> >>independently of the musl internal implementation of threads.
> >>Thus when clone() in musl/src/linux/clone.c calls
> >> return __syscall_ret(__clone(func, stack, flags, arg, ptid, tls, ctid));
> >>then the i386 implementation of __clone has no guarantee about
> >>the value in %gs, and it is a bug to assume that (%gs >> 3)
> >>fits in 8 bits.
> >
> >The ABI is that at function call or any time a signal could be
> >received, %gs must always be a valid segment register value reflecting
> >the current thread's thread pointer. If this is violated, the program
> >has undefined behavior.
>
> More than one segment descriptor can designate the same subset
> of the linear address space. Duplicate the segment descriptor
> to a target selector that is >= 256, and load %gs with the
> duplicate selector before calling clone().
It's not clear to me that such a substition is valid; as far as I can
tell no explicit effort to ensure that it works is made, and it would
not happen without writing asm to do specifically that.
> >>The code in musl/src/thread/i386/clone.s wastes up to 12 bytes
> >>when aligning the new stack, by aligning before [pre-]allocating
> >>space for the one argument to the thread function.
> >
> >I suspect the initial value happens to be aligned anyway in which case
> >reserving 16 bytes and aligning to 16 is the same as reserving 4 and
> >aligning to 16. If you think it's not, I don't mind changing if you
> >can do careful testing to make sure it doesn't introduce any bugs.
>
> This is another bug! Consider the valid code:
> void **lo_stack = malloc(5 * sizeof(void *));
> /* malloc() guarantees 16-byte alignment of lo_stack */
> clone(func, &lo_stack[5], ...);
You can't run code on a 20-byte stack. This is not a surprise. In
theory it might be possible if the callee is only asm, but you can't
make C function calls since each call frame will consume at least 16
bytes (return address and alignment). I also disagree with considering
it valid to assume clone invokes the provided callback function
directly with no intervening functions; this is incorrect on SH right
now since we use a C function to smooth over the difference between
plain and fdpic calling conventions. ARM will probably do the same
once fdpic for cortex-M is added.
> then __clone() does:
> and $-16,%ecx /* &lo_stack[4] */
> sub $ 16,%ecx /* &lo_stack[0] */
> ...
> mov %ecx,%esp /* new thread: implicit action of ___NR_clone system call */
> call *%eax /* OUT-OF-BOUNDS: lo_stack[-1] = return address */
>
> Thus, starting the thread function has scribbled outside the allocated area,
> even though the lo_stack[] array can accommodate the call by the code I showed:
> lea -NBPW(arg2),%ecx /* &lo_stack[4] */
> and $-16,%ecx /* still &lo_stack[4] */
> ...
> mov %ecx,%esp /* new thread: implicit action of __NR_clone system call */
> call *%eax /* lo_stack[3] = return address */
>
> The danger is not "new bugs", but rather revealing latent bugs that were
> obscured by the less-strict old code. For instance, if the thread
> function actually has two formal parameters, or if it uses va_arg()
> to reference beyond the first actual argument, then running the optimal
> code is more likely to notice.
I agree with your analysis of what happens but I don't think it's
particularly interesting or a bug.
Rich
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