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I've upgraded my Linux development VM from Ubuntu 16.04 to 18.04 recently, and noticed one thing that has changed. This is on x86-64. With 16.04, I've always had this workflow where I'd build the project I'm working on with gcc (5.4, the stock version in 16.04) and -fsanitize=address and -O0 -g, and then run the executable through gdb (7.11.1, also the version that came with Ubuntu). This worked fine, and at the end, LeakSanitizer would produce a leak report if it detected memory leaks.

In 18.04, this doesn't seem to work anymore; LeakSanitizer complains about running under ptrace:

==5820==LeakSanitizer has encountered a fatal error.
==5820==HINT: For debugging, try setting environment variable LSAN_OPTIONS=verbosity=1:log_threads=1
==5820==HINT: LeakSanitizer does not work under ptrace (strace, gdb, etc)

Then the program crashes:

Thread 1 "spyglass" received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51

I'm not sure what is causing the new behavior. On 18.04 I'm building with the default gcc shipped (7.3.0), using -fsanitize=address -O0 -g and debugging with the default gdb (8.1.0). Can the old behavior be somehow re-enabled? Or do I need to change my workflow and detach from the program before killing it to get a leak report?

fencekicker
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    In the meantime I found this: https://github.com/google/sanitizers/issues/857 . Quite short on info, though. – fencekicker Jan 04 '19 at 09:19
  • I traced things on a small sample program. It appears that the lsan in gcc 5 doesn't consider the ptrace failure to be fatal. I don't yet know what difference that ignored failure makes in the quality of its results, but I'll look. Workaround is to install the `gcc-5` package on your 18.04 system (it's in the standard repo) and use `gcc-5` everywhere you'd use `cc`. – Mark Plotnick Jan 04 '19 at 21:57

1 Answers1

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LeakSanitizer internally uses ptrace, probably to suspend all threads such that it can scan for leaks without false positives (see issue 9). Only one application can use ptrace, so if you run your application under gdb or strace, then LeakSanitizer won't be able to attach via ptrace.

If you are not interested in leak debugging, disable it:

export ASAN_OPTIONS=detect_leaks=0

If you do want to enable leak debugging, you must detach the debugger before LeakSanitizer starts scanning. To be able to attach a debugger shortly afterwards, sleep a bit (for example, 10 seconds):

export ASAN_OPTIONS=sleep_before_dying=10
./program

Then in another shell, attach to the application again:

gdb -q -p $(pidof program)

For more a description of the above (and other) options, see https://github.com/google/sanitizers/wiki/AddressSanitizerFlags.

Lekensteyn
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  • Thanks, I noticed that leak checking can be disabled. It's still annoying, though, this worked perfectly in the past. – fencekicker Jan 31 '19 at 09:06
  • Note you have a typo in the above line, at least if running under bash. The export XXX:= syntax does not work: export ASAN_OPTIONS:sleep_before_dying=1 bash: export: `ASAN_OPTIONS:sleep_before_dying=1': not a valid identifier – fencekicker Jan 31 '19 at 09:06
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    Fixed the typo. Usually I have more colon-separated options such as: `ASAN_OPTIONS=abort_on_error=1:report_objects=1:sleep_before_dying=1`. – Lekensteyn Jan 31 '19 at 11:31