linux-kernel - Re: linux interprets an fcntl int arg as long

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Message-ID: <Y2DisyknbKxeCik4@arm.com>
Date:   Tue, 1 Nov 2022 09:11:15 +0000
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: linux interprets an fcntl int arg as long

The 10/31/2022 21:46, Theodore Ts'o wrote:
> 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.

the callee uses va_arg(ap, type) to get the argument,
and if the type is wider than what was actually passed
then anything can happen. in practice what happens is
that the top bits can be non-zero.

many pcs are affected (aarch64 is the one i know well,
but at least x86_64, arm are affected too). and even if
it was aarch64 pcs only, it is incompetent to say that
the pcs is wrong: that's a constraint we are working with.

the kernel must not read a wider type than what it
documents as argument to variadic functions in the c api.
(it does not make much sense to expect anything there
anyway, but it can break userspace)