linux-kernel - Compat sysinfo syscall (kernel/sys.c) relying on undefined behavior?

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Message-ID: <54062A22.6090804@eng.utah.edu>
Date:	Tue, 02 Sep 2014 14:35:46 -0600
From:	Scotty Bauer <sbauer@....utah.edu>
To:	linux-kernel@...r.kernel.org
Subject: Compat sysinfo syscall (kernel/sys.c) relying on undefined behavior?

 am getting acquainted with the linux kernel and to do so I've been browsing the source.

In the compat version of sysinfo, kernel/sys.c we see the following:

COMPAT_SYSCALL_DEFINE1(sysinfo, struct compat_sysinfo __user *, info)
{
    struct sysinfo s;

    do_sysinfo(&s);

    /* Check to see if any memory value is too large for 32-bit and scale
     *  down if needed
     */
    if ((s.totalram >> 32) || (s.totalswap >> 32)) {
        int bitcount = 0;

...

s.totalram is a u32, and the standard says:
"If the value of the right operand is negative or is greater than or equal to the width 
of the promoted left operand, the behavior is undefined."

Is there some promotion, compiler flag, something obvious that I am missing, or is this a 
problem?

Best,
Scotty

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