Re: linux interprets an fcntl int arg as long

From: "Theodore Ts'o" <tytso@mit.edu>
To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: linux interprets an fcntl int arg as long
Date: Mon, 31 Oct 2022 21:46:53 -0400	[thread overview]
Message-ID: <Y2B6jcLUJ1F2y2yL@mit.edu> (raw)
In-Reply-To: <Y1/DS6uoWP7OSkmd@arm.com>

On Mon, Oct 31, 2022 at 12:44:59PM +0000, Szabolcs Nagy wrote:
> and such fcntl call can happen with c code that just passes
> F_SEAL_WRITE since it is an int and e.g. with aarch64 pcs rules
> it is passed in a register where top bits can be non-zero
> (unlikely in practice but valid).

In Linux's aarch64 ABI, an int is a 4-byte value.  It is *not* an
8-byte value.  So passing in "F_SEAL_WRITE | 0xF00000000" as an int
(as in your example) is simply not valid thing for the userspace
program to do.

Now, if there is a C program which has "int c = F_SEAL_WRITE", if the
PCS allows the compiler to pass a function paramter c --- for example
f(a, b, c) --- where the 4-byte paramter 'c' is placed in a 64-bit
register where the high bits of the 64-bit register contains non-zero
garbage values, I would argue that this is a bug in the PCS and/or the
compiler.

						- Ted