Re: [RFC][PATCH 0/4 v2] sycalls: Remove args i and n from syscall_get_arguments()

["Dmitry V. Levin" <ldv@altlinux.org>]

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On Fri, Mar 29, 2019 at 11:12:18AM -0700, Linus Torvalds wrote:
> On Fri, Mar 29, 2019 at 10:40 AM Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > I'll keep it around for now, but this should go as a warning to Dmitry,
> > to get something using it soon, or they may be dropped.
> 
> I don't think _that_ is the argument.
> 
> Quite the reverse: nobody has ever used it, why have it around, and
> much less try to hurry some new pointless user to use it?
> 
> The "get system call arguments" code at least can be used somewhat
> generically for things like tracing and strace.
> 
> The "set system call arguments" can NOT.
> 
> Anybody who sets system call arguments had better intimately know the
> details anyway, and any user code has to have any legacy ptrace
> interface anyway for all but the newest kernels.

In strace we have a feature called system call tampering.
Initially limited to system call number and return code tampering,
it's being extended to tamper with system call arguments as well.

Currently it's implemented in strace using traditional
PTRACE_SETREGSET/PTRACE_SETREGS/PTRACE_POKEUSER interfaces.
These interfaces indeed require intimate knowledge of the target
architecture.  Fortunately, strace already has this intimate knowledge,
but the corresponding code would be much more trivial if an
architecture-agnostic ptrace interface for setting syscall info
existed in the kernel.

I didn't plan to start the discussion about this new ptrace command
before PTRACE_GET_SYSCALL_INFO [1] finally landed into the kernel.

For us userspace people it takes a lot of time not only to get a new
kernel interface accepted, but even to reintroduce an old internal kernel
interface that was removed due to lack of users.  For example, it took me
roughly 4 months to get a relatively simple partial revert of commit
5e937a9ae913 accepted into linux-next.

This was the reason why I asked to delay the removal of
syscall_set_arguments() until PTRACE_GET_SYSCALL_INFO
is merged into the kernel.

[1] https://lore.kernel.org/lkml/20190322041409.GA27266@altlinux.org/


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ldv

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