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Date: Thu, 21 Feb 2019 15:28:13 +0100
From: Jann Horn <>
To: Kees Cook <>
Cc: "Tobin C. Harding" <>, Shuah Khan <>, 
	Alexander Shishkin <>, 
	Greg Kroah-Hartman <>, 
	Andy Shevchenko <>, 
	Kernel Hardening <>, 
	kernel list <>, Andy Lutomirski <>, 
	Rasmus Villemoes <>, Daniel Micay <>
Subject: Re: [PATCH 5/6] lib: Fix function documentation for strncpy_from_user

On Thu, Feb 21, 2019 at 2:05 AM Kees Cook <> wrote:
> On Mon, Feb 18, 2019 at 4:52 PM Jann Horn <> wrote:
> > AFAICS the byte_at_a_time loop exits when max==0 is reached, and then
> > if `res >= count` (in other words, if we've copied as many bytes as
> > requested, haven't encountered a null byte so far, and haven't reached
> > the end of the address space), we return `res`, which is the same as
> > `count`. Are you sure?
> Oh, whew, there is only 1 arch-specific implementation of this. I
> thought you meant there was multiple implementations.

Not sure what you're talking about. Are you talking about
implementations of strncpy_from_user()?

csky: custom strncpy_from_user() calls custom __do_strncpy_from_user,
which is a macro containing a big asm() block.

;;; long strncpy_from_user(void *to, void *from, size_t n)
mov.l er2,er2
bne 1f
sub.l er0,er0

static inline long
strncpy_from_user(char *__to, const char __user *__from, long __len)
long res;

if (eva_kernel_access()) {
__asm__ __volatile__(
"move\t$4, %1\n\t"
"move\t$5, %2\n\t"
"move\t$6, %3\n\t"

mov ip, r1
1: sub.a r2, r2, #1
ldrusr r3, r1, 1, ns
bfs 2f
stb.w r3, [r0]+, #1
cxor.a r3, #0

and so on.

> So, generally speaking, I'd love to split all strncpy* uses into
> strscpy_zero() (when expecting to do str->str copies), and some new
> function, named like mempadstr() or str2mem() that copies a str to a
> __nonstring char array, with trailing padding, if there is space. Then
> there is no more mixing the two cases and botching things.

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