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Message-ID: <20230602152327.GT4163@brightrain.aerifal.cx>
Date: Fri, 2 Jun 2023 11:23:27 -0400
From: Rich Felker <dalias@...c.org>
To: Markus Wichmann <nullplan@....net>
Cc: musl@...ts.openwall.com
Subject: Re: __convert_scm_timestamps() probably breaks with struct
timeval
On Fri, Jun 02, 2023 at 04:49:37PM +0200, Markus Wichmann wrote:
> Hi all,
>
> I couldn't think of a better subject. For unrelated reasons, today I
> stumbled upon the abovenamed function. It looks through all control
> messages in a msghdr, looking for SCM_TIMESTAMP_OLD and
> SCM_TIMESTAMPNS_OLD, and converting an array two long into an array of
> two long long.
>
> This will work with struct timespec (or SCM_TIMESTAMPNS_OLD) on all
> architectures, because struct timespec has been defined specifically for
> this to work (due to the kernel accessing it the same way). But it will
> break with struct timeval (or SCM_TIMESTAMP_OLD).
>
> First of all, on some 32-bit archs (and this code is only used on 32-bit
> archs) we have alignof(int64_t) == 4, and therefore sizeof (struct
> timeval) == 12. Therefore CMSG_LEN(sizeof (long long[2])) !=
> CMSG_LEN(struct timeval), and therefore, applications won't read the
> newly converted cmsg. All the work for nothing.
>
> Next, even if alignof(int64_t) == 8, struct timeval does not have the
> same padding as struct timespec, and so on big endian architectures, the
> code ends up writing the microseconds into the padding and clearing the
> tv_usec field.
>
> This can all be solved easily, if you just try to be a touch less
> clever. I am attaching a patch that makes the intent more obvious. It's
> not as flashy as the previous code. It doesn't have a switch with a goto
> in it. I hope it finds your favor.
I'm confused what problem you're seeing. There is no conversion
in-place. The function finds a timeval or timespec in the old cmsg
array, extracts the value to a local variable that's a pair of long
long, and finally *appends* that as a new cmsg to the cmsg array.
Rich
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