Re: [PATCH 2/6] exec: simplify the compat syscall handling

Arnd Bergmann
2020-06-15T13:31:35Z

On Mon, Jun 15, 2020 at 3:00 PM Christoph Hellwig <hch@lst.de> wrote:
>
> The only differenence betweeen the compat exec* syscalls and their
> native versions is that compat_ptr sign extension, and the fact that
> the pointer arithmetics for the two dimensional arrays needs to use
> the compat pointer size. Instead of the compat wrappers and the
> struct user_arg_ptr machinery just use in_compat_syscall() to do the
> right thing for the compat case deep inside get_user_arg_ptr().
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Nice!

I have an older patch doing the same for sys_mount() somewhere,
but never got around to send that. One day we should see if we
can just do this for all of them.

> -
> -static const char __user *get_user_arg_ptr(struct user_arg_ptr argv, int nr)
> +static const char __user *
> +get_user_arg_ptr(const char __user *const __user *argv, int nr)
> {
> const char __user *native;
>
> #ifdef CONFIG_COMPAT
> - if (unlikely(argv.is_compat)) {
> + if (in_compat_syscall()) {
> + const compat_uptr_t __user *compat_argv =
> + compat_ptr((unsigned long)argv);
> compat_uptr_t compat;
>
> - if (get_user(compat, argv.ptr.compat + nr))
> + if (get_user(compat, compat_argv + nr))
> return ERR_PTR(-EFAULT);
>
> return compat_ptr(compat);
> }
> #endif

I would expect that the "#ifdef CONFIG_COMPAT" can be removed
now, since compat_ptr() and in_compat_syscall() are now defined
unconditionally. I have not tried that though.

> +/*
> + * x32 syscalls are listed in the same table as x86_64 ones, so we need to
> + * define compat syscalls that are exactly the same as the native version for
> + * the syscall table machinery to work. Sigh..
> + */
> +#ifdef CONFIG_X86_X32
> COMPAT_SYSCALL_DEFINE3(execve, const char __user *, filename,
> - const compat_uptr_t __user *, argv,
> - const compat_uptr_t __user *, envp)
> + const char __user *const __user *, argv,
> + const char __user *const __user *, envp)
> {
> - return do_compat_execve(AT_FDCWD, getname(filename), argv, envp, 0);
> + return do_execveat(AT_FDCWD, getname(filename), argv, envp, 0, NULL);
> }

Maybe move it to arch/x86/kernel/process_64.c or arch/x86/entry/syscall_x32.c
to keep it out of the common code if this is needed. I don't really understand
the comment, why can't this just use this?

--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -379,7 +379,7 @@
517 x32 recvfrom compat_sys_recvfrom
518 x32 sendmsg compat_sys_sendmsg
519 x32 recvmsg compat_sys_recvmsg
-520 x32 execve compat_sys_execve
+520 x32 execve sys_execve
521 x32 ptrace compat_sys_ptrace
522 x32 rt_sigpending compat_sys_rt_sigpending
523 x32 rt_sigtimedwait compat_sys_rt_sigtimedwait_time64
@@ -404,7 +404,7 @@
542 x32 getsockopt compat_sys_getsockopt
543 x32 io_setup compat_sys_io_setup
544 x32 io_submit compat_sys_io_submit
-545 x32 execveat compat_sys_execveat
+545 x32 execveat sys_execveat
546 x32 preadv2 compat_sys_preadv64v2
547 x32 pwritev2 compat_sys_pwritev64v2
548 x32 process_madvise compat_sys_process_madvise

Arnd