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Date: Tue, 24 Nov 2015 20:54:28 +0100
From: Florian Weimer <>
Subject: Re: System call interface changes

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.

>> 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?

But I appreciate your comments, they have been helpful.


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