Difference between revisions of "Debugging notes"
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== Debugging with LD_PRELOAD hooks == | == Debugging with LD_PRELOAD hooks == | ||
+ | See http://michael.toren.net/slides/lkm-alternatives/slide006.html | ||
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+ | [[LD_PRELOAD_notes]] | ||
== Debugging with `strace` == | == Debugging with `strace` == |
Revision as of 00:29, 7 May 2010
Debugging with LD_PRELOAD hooks
See http://michael.toren.net/slides/lkm-alternatives/slide006.html
Debugging with `strace`
For these examples, I usually open two xterm windows for experimentation. In one shell I use `strace` to attach to the PID of the other shell. You can get the PID of a shell by running `echo $$` in the shell you want to watch.
This will show all file related activity by the process, $PID. Note that -e trace=file is synonymous with -e file. The man page for `strace` explains the expression syntax for the -e option.
strace -p $PID -f -e trace=file
You can use the -c option to get a count of each system call made by the process. The count is made after filtering by the -e expression. The count is printed when `strace` exits. The following will show a count of each file related system call.
strace -p $PID -c -f -e trace=file
This will show a count of all system calls.
strace -p $PID -c -f
This will show what data is written by the process, $PID.
strace -p $PID -f -e write
The '-f' option tells `strace` to follow children that are forked and execed. Note that this is a best effort and that strace can miss a few system calls of the child while it starts up. This can be significant in the real world.
The following will trace data that is read and written by the process, $PID:
strace -f -e write,read -p $PID
The following will trace data that is written by the process, $PID. This is useful for tracing stdout of shells and the like. I don't know why I need both -e options. I think I should need only -e write=2, but if I do that then I see every other system call. Nor can I do -e write,write=2. This syntax gives me a headache... At any rate, this mostly does what I want, but not quite -- it does not actually print re response back from the shell in the hex dump:
strace -f -s 1024 -e write -e write=2 -p $PID
This will show files created by a process. Note that files can be created and opened for writing using 'creat' as well as 'open'. Here I filter a lot of the open calls. The "-o /proc/self/fd/1" forces output to stdout.
strace -o /proc/self/fd/1 -p $PID -f -e creat,open | grep -v O_RDONLY
In theory, you could also force output to stdout with '-o \|cat', but piping through cat seems to take more time, so `strace` misses more child calls when it tried to follow them. It is also slower to use `strace's` own built-in filter option '-e'. It is faster to pipe through grep for later filtering. For example, this will often miss 'open' calls to open files for writing:
strace -o \|cat -p $PID -f -e creat -e open | grep -v O_RDONLY
But will work a little better:
strace -o \|cat -p $PID -f | grep -v O_RDONLY | grep open
You might miss a file open or creat when using redirects from the parent shell. If you use this in a command-line pipe stream that the process you trace has the file opened for it already as file descriptor 1. This should be no surprise...
strace -f -e write echo foo > foo.txt