RE: [RFC][PATCH 01/22] x86 user stack frame reads: switch to explicit __get_user()

[David Laight]

From: Al Viro
> Sent: 29 March 2020 18:58
...
> [*] IMO compat_alloc_user_space() should die; this "grab some space on
> user stack, copy the 32bit data structure into 64bit equivalent there,
> complete with pointer chasing and creating 64bit equivalents of everything
> that's referenced from that struct, then call native ioctl, then do the
> reverse conversion" is just plain wrong.  That native ioctl is going to
> bring the structures we'd constructed back into the kernel space and
> work with them there; we might as well separate the function that work
> with the copied struct (usually we do have those anyway) and call those
> instead the native ioctl.  And skip the damn "copy the structures we'd
> built into temp allocation on user stack, then have it copied back"
> part.  We have relatively few callers, thankfully.

I helped rip the same 'stackgap' code out of netbsd many years ago.
No only was it being used for system call compatibility, but
also for security checks and rewriting filenames.
Completely hopeless in a threaded program.

	David

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