From: Andrew Morton <akpm@linux-foundation.org>
To: Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>
Cc: linux-mm@kvack.org, rostedt@goodmis.org, mhiramat@kernel.org,
andrii@kernel.org, kernel-team@meta.com,
linux-kernel@vger.kernel.org, Mykyta Yatsenko <yatsenko@meta.com>
Subject: Re: [PATCH mm] maccess: fix strncpy_from_user_nofault empty string handling
Date: Thu, 17 Apr 2025 13:40:06 -0700 [thread overview]
Message-ID: <20250417134006.60e0d6b3fc963bcbec1255b0@linux-foundation.org> (raw)
In-Reply-To: <20250417152808.722409-1-mykyta.yatsenko5@gmail.com>
On Thu, 17 Apr 2025 16:28:08 +0100 Mykyta Yatsenko <mykyta.yatsenko5@gmail.com> wrote:
> strncpy_from_user_nofault should return the length of the copied string
> including the trailing NUL, but if the argument unsafe_addr points to
> an empty string ({'\0'}), the return value is 0.
>
> This happens as strncpy_from_user copies terminal symbol into dst
> and returns 0 (as expected), but strncpy_from_user_nofault does not
> modify ret as it is not equal to count and not greater than 0, so 0 is
> returned, which contradicts the contract.
Looks right, I think.
But why do strncpy_from_user() and strncpy_from_user_nofault() have
different interfaces?
/**
* strncpy_from_user: - Copy a NUL terminated string from userspace.
* ...
* On success, returns the length of the string (not including the trailing
* NUL).
/**
* strncpy_from_user_nofault: - Copy a NUL terminated string from unsafe user
* address.
* ...
* On success, returns the length of the string INCLUDING the trailing NUL.
This is surprising and I'm wondering what led us to do this?
next prev parent reply other threads:[~2025-04-17 20:40 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-04-17 15:28 Mykyta Yatsenko
2025-04-17 20:40 ` Andrew Morton [this message]
2025-04-17 21:07 ` Andrii Nakryiko
2025-04-17 20:44 ` Andrii Nakryiko
2025-04-17 20:47 ` Andrii Nakryiko
2025-04-17 22:10 ` Steven Rostedt
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