gdb - debugging with piped input (not arguments)

Question

I typically run my program with:

perl -e 'print "A"x200' | ./vuln_prog

The stdin is consumed by a gets() function in C++.

If this were just a command argument, I could open gdb by doing:

gdb ./vuln_prog
run $(perl -e 'print "A"x200')

However, my scenario is not a command argument, but rather input (STDIN?). How would I debug this in gdb? I've tried a bunch of options, but nothing seems to really work.

I would normally just run gdb on the process, and when it prompts for user input, type it in, however I'm not wanting to type just "A". I want to type all chars from \x00-\xff, which I can't type.

Why are you using `gets`? I mean sometimes it's acceptable (like if you're just writing a quick program to test something, or if the program will only be run with trusted input) but I'm curious. — flarn2006, Sep 19 '16 at 22:33
It was not my code. It was for a fun reverse engineering challenge, where the code was provided. — mandreko, Sep 20 '16 at 12:28
Oh okay. Was `gets` intentionally used *because* it was vulnerable, like as part of a possible solution to the challenge? — flarn2006, Sep 21 '16 at 02:38
@flarn2006 Indeed it was the vulnerable function. I was able to do a buffer overflow due to it not bound-checking. — mandreko, Sep 21 '16 at 14:02

score 118 · Accepted Answer · answered Dec 07 '11 at 20:47

118

gdb ./vuln_prog
run < filename_with_input

answered Dec 07 '11 at 20:47

zed_0xff

31,180
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Thank you SO much. I was trying to run the input directly, where I should have thought to just print the perl code to a file, and input it. That worked marvelously. – mandreko Dec 07 '11 at 20:57
2

Does the have a `lldb` equivalent? – Thomas Ahle Jan 03 '16 at 20:57
2

@ThomasAhle For lldb equivalent http://stackoverflow.com/questions/29861242/cannot-get-mac-os-x-lldb-process-to-read-the-stdin – jernkuan Jul 18 '16 at 10:20
1

What if I need to run a command as an input? Like `cat file.bz2 | bunzip2 | myProgramThatNeedsDebugging` ? – iAdjunct May 10 '18 at 14:10
@iAdjunct, in that case do "bunzip2 < file.bz2 > filename_with_input" before running gdb. – j13r May 31 '18 at 01:40
@j13r Ha, sorry, I'm good at finding ridiculous cases. My bzip'd file is ~500MB and, unzipped, it's roughly 80GB (file paths, lots and lots of file paths). I can't store them unzipped. – iAdjunct May 31 '18 at 03:18
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@iAdjunct I saw in another question that you can use the bash syntax, i.e., "run < – j13r May 31 '18 at 16:50
5

is there any way to pipe without writing to file? – Shinlos Apr 22 '19 at 17:58
1

@Shinlos I'm still trying to figure out how this syntax works, but I believe, `run < – Josh Desmond Apr 02 '20 at 18:27
@JoshDesmond it does indeed work this way, I just had to change it to `run < – diego92sigma6 Sep 13 '21 at 21:02

gdb - debugging with piped input (not arguments)

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