A sys_poll() ABI tweak

The poll() system call has three parameters, one of which is a timeout value specifying an upper bound (in milliseconds) for how long the process will wait. The manual page indicates that the type of this value is int. For reasons lost in history, though, the kernel's internal implementation of poll() has always expected the timeout value to be a long integer. And that has created a source of occasional bugs.

Most of the time, things just work. The int and long types tend to be the same on most architectures, and, in cases where they are different, glibc sign-extends the timeout value appropriately. Things go wrong, though, when a 32-bit process is running on an x86-64 system. In that case, the 32-bit sys_poll() function just passes the timeout value directly to the native kernel version, without sign extension. So if the timeout value is negative (an indication that poll() should wait forever if need be), the kernel will eventually see a large, positive timeout instead.

There are various ways this problem could be fixed. What Linus has chosen to do, though, is to just change the type of the timeout parameter to int inside the kernel. Since the timeout is now a 32-bit quantity on all systems, that particular source of confusion is removed. There is a small risk to this approach, though: it is possible that some program somewhere was actually making use of 64-bit timeouts. Doing so would require replacing or bypassing glibc (because its sign extension makes 64-bit timeouts unusable), so it's unlikely that anybody has bothered, but one never knows. If this change were to break a real application, it would have to be reverted in favor of a more complicated solution.

Linus's patch was merged for 3.3-rc5, so anybody who objects has a few weeks to make their concerns known.

Index entries for this article
Kernel	Development model/User-space ABI

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