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Date: Wed, 20 Feb 2019 17:05:10 -0800
From: Kees Cook <>
To: Jann Horn <>
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 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.

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.

Kees Cook

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