How to debug functions in a dynamic library loaded with LD_PRELOAD with gdb?

Question

I'm trying to debug some functions in a dynamic shared library libexecHook.so. This library is preloaded setting LD_PRELOAD in order to intercept and rewrite some calls to execve() and friends. For debugging purposes I have built gmake with symbols. From what I read in other questions this should work:

gdb ~/tmp/make-dfsg-3.81/make
set exec-wrapper env LD_PRELOAD=/home/marko/execHook.C027/lib/libexecHook.so.0.0.0
start
break execve
break execvp
cont

I do see the breakpoints being set properly, e.g.

4       breakpoint     keep y   0x00007ffff7bd92e0 in execvp at execHook.c:128

but gdb never breaks at my preloaded exec..() functions. Watching the debug output during execution I see that the my library functions are being called.

Have you tried also setting LD_PRELOAD before launching gdb? — John Zwinck, Feb 10 '12 at 15:25
I hadn't tried it before, but it does not make a difference. — Marko, Feb 10 '12 at 17:39

score 3 · Accepted Answer · answered Feb 11 '12 at 16:39

The reason gdb did not break at my preloaded wrapper functions is that these are executed from a child process to which gdb was not attached. On Linux I can

set follow-fork-mode child

to make gdb attach to the child that gets created in a vfork().

score 1 · Answer 2 · answered Feb 09 '12 at 22:14

1

Try saying start before setting your breakpoints. This will begin running the program which will cause the library dependencies to be satisfied, hopefully using your LD_PRELOAD path. Then set the breakpoints after start, and then do continue.

answered Feb 09 '12 at 22:14

John Zwinck

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The exact same thing: I see the breakpoints: `4 breakpoint keep y 0x00007ffff7bd92e0 in execvp at execHook.c:128` and the program's debug output shows that it's using the functions, but the breakpoints are never triggered. – Marko Feb 10 '12 at 11:02

How to debug functions in a dynamic library loaded with LD_PRELOAD with gdb?

2 Answers2