SIGALRM blocked forever — by init

September 23rd, 2011 by Ralf Schlatterbeck

I’m working on an embedded Linux system which allows to chose the root filesystem to boot from (flash card or NAND flash) early in the boot process. Now I was trying to get ppp (for a GSM/GPRS connection) working. But the chat-script hangs forever, it does not get a timeout. Turns out, chat uses alarm(2) to wait for a timeout. After quite some time of debugging I found out that SIGALRM is blocked (look for SigBlk in /proc/pid/status, this is the mask of blocked signals for process with pid).

After some googling I came across a blog entry (look for September 5, 2011) that describes a bug in bash: Bash, when calling “read” with a timeout, will install a signal handler for SIGALRM and longjump out of the signal handler, leaving SIGALRM blocked forever.

The boot-script runs as init (with pid 1) and therefore will leave SIGALRM blocked for all children. My immediate workaround is to read the filesystem to boot in a sub-shell…seems the bug is fixed with newer versions of bash.

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