fgets gets stuck into an infinite loop while reading /proc/PID/maps

Question

I'm trying to read from /proc/<PID>/maps file. I ptrace(PTRACE_ATTACH, <PID>) before reading (so, the process is supposed to be suspended).

The code that reads looks like this:

while(fgets(line_buf, BUFSIZ, maps_fd) != NULL){ ... }

but it's not ending (infinite loop). I did a printf line_buf and in turns out fgets it reading lines that it has read already.

Questions: What could be changing my maps file? (if anything is changing it)

If the file ins't changed by something else, why is fgets reading lines that it has read already?

score 1 · Accepted Answer · answered Oct 17 '14 at 12:30

1

I can't find the reference right now, but this was a known bug/regression in certain kernel versions. The problem is almost surely that you're running on a buggy kernel and need to replace it with a less-buggy one. If I can find a link to the issue report I'll add it.

answered Oct 17 '14 at 12:30

R.. GitHub STOP HELPING ICE

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Ok, I tried updating to the very latest kernel `[alexandernst@stupidbox 0s ~]$ uname -a Linux stupidbox 3.16.4-1-ARCH #1 SMP PREEMPT Mon Oct 6 08:22:27 CEST 2014 x86_64 GNU/Linux` but I'm still getting the same. Has that bug been patched? – alexandernst Oct 17 '14 at 13:35
I sure thought so. Does it only happen when tracing? Perhaps you're doing something wrong with `ptrace`. Or perhaps it's a `ptrace`-specific bug. – R.. GitHub STOP HELPING ICE Oct 17 '14 at 15:09
Uh, actually, forget that, it did fix after upgrading – alexandernst Oct 17 '14 at 16:39
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Can you report which version you were using that was broken? That might make this question more helpful to people who find it in the future. – R.. GitHub STOP HELPING ICE Oct 17 '14 at 17:12
And it might also help me search for documentation of the bug and when it was fixed. So far searches have just turned up junk. – R.. GitHub STOP HELPING ICE Oct 17 '14 at 19:38
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I was using 3.15.2 and updated to 3.16.4, so the bug must be somewhere in between those two versions. – alexandernst Oct 18 '14 at 10:53

fgets gets stuck into an infinite loop while reading /proc/PID/maps

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