From: Szabolcs Nagy <szabolcs.nagy@arm.com>
To: "Theodore Ts'o" <tytso@mit.edu>
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: Tue, 1 Nov 2022 09:11:15 +0000 [thread overview]
Message-ID: <Y2DisyknbKxeCik4@arm.com> (raw)
In-Reply-To: <Y2B6jcLUJ1F2y2yL@mit.edu>
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)
next prev parent reply other threads:[~2022-11-01 9:11 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-31 12:44 linux interprets an fcntl int arg as long Szabolcs Nagy
2022-11-01 1:46 ` Theodore Ts'o
2022-11-01 9:11 ` Szabolcs Nagy [this message]
2022-11-01 10:02 ` David Laight
2022-11-01 11:44 ` 'Szabolcs Nagy'
2022-11-01 12:19 ` David Laight
2022-11-01 12:49 ` 'Szabolcs Nagy'
2022-11-01 13:12 ` Mark Rutland
2022-11-01 13:29 ` David Laight
2022-11-01 13:35 ` David Laight
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=Y2DisyknbKxeCik4@arm.com \
--to=szabolcs.nagy@arm.com \
--cc=linux-api@vger.kernel.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=tytso@mit.edu \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).