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Date: Tue, 24 Nov 2015 15:29:35 -0500
From: Rich Felker <>
Subject: Re: System call interface changes

On Tue, Nov 24, 2015 at 08:54:28PM +0100, Florian Weimer wrote:
> On 11/20/2015 08:16 PM, Rich Felker wrote:
> >> This would have to be an opt-in feature, obviously, and applications
> >> would have to opt in explicitly via some ELF flag (similar to what we
> >> did for non-executable stacks).
> > 
> > I don't think that's necessary. The application (or for typical
> > dynamic linking, just the build of would just need to refrain
> > from using the parameterized syscall so that the old opcode would not
> > appear in its executable mappings.
> The SYSCALL instruction is fairly short (0x0f 0x05), so it ends up in
> process images by accident.  I think this calls for explicit blocking.

Indeed, that makes sense at least for x86 where you have misaligned
instructions and immediates in the instruction stream.

> >> Do you think it would be feasible to encode the system call number in
> >> the instruction stream instead, next to the instruction?  I think this
> > 
> > This was done on ARM in the old pre-EABI ABI, and it turned out to be
> > a bad design, at least from standpoints other than security. Reading
> > the syscall number out of the instruction stream was more expensive,
> > incompatible with syscall() (which ended up requiring a special
> > SYS_syscall that needed messy argument conventions), and incompatible
> > with reasonable userspace coding of syscalls using inline functions
> > rather than macros, where you would have to rely on constant
> > propagation optimizations to be able to satisfy asm constraints.
> Wouldn't it be possible to embed the constant in the assembly text,
> using the C preprocessor?

Not unless you're using macros all the way down, with no inline
functions. I suppose that's a choice glibc could make, but I'd
consider it a bad choice in terms of style and flexibility. Inline
functions are preferable IMO, but they don't allow you to propagate
the constant expression semantic (or string literals) from the caller
into the callee.

BTW, as long as has the syscall() function, there's going to
be at least one way to make arbitrary syscalls by jumping to a
particular address with the right values in the right registers or
stack slots.


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