How to run valgrind on a CGI program in C
There was still one bug left with mod_blog [1]—it would crash with a memory corruption error (thanks to checking in glibc [2] when doing a POST. I only found the bug because I was using the old web interface to make sure I had the right credentials when testing the PUT method [3]. How long had the bug existed? At least six years—it's been seven sine I last used the web interface (I checked).
It did not crash on the development server due to subtle differences between the operating system and versions of glibc being used. But it is ultimately a memory corruption, so the use of valgrind [4] would be instrumental in finding the issue. The problem is—it only manefests itself when doing a POST, which means testing the program under a web server. And a web server will pass information about the request to the CGI (Common Gateway Interface) program through environment variables, and any input comes in via stdin.
So just how do you run valgrind on a program meant to be run as a CGI program?
After some thought, I figured out a way. I need to capture the environment the CGI program runs under, so I added the following bit of code to mod_blog to capture the environment:
extern char *envriron;
FILE *fp = fopen("/tmp/env.txt","w");
for (size_t i = 0 ; environ[i] != NULL ; i++)
fprintf(fp,"export %s\n",environ[i]);
fclose(fp);
I wasn't worried about error checking—this is temporary code anyway. I then do a POST and I now have the environment variables in a file:
... export GATEWAY_INTERFACE=CGI/1.1 export HTTP_ACCEPT=text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 export CONTENT_LENGTH=149 export CONTENT_TYPE=application/x-www-form-urlencoded export REQUEST_METHOD=POST ...
The reason I added “export” was to copy these environment variables to a shell script where I can then run valgrind and debug the situation:
... export GATEWAY_INTERFACE=CGI/1.1 export HTTP_ACCEPT=text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,\*/\*\;q=0.8 export CONTENT_LENGTH=149 export CONTENT_TYPE=application/x-www-form-urlencoded export REQUEST_METHOD=POST ... valgrind $HOME/source/boston/src/main <r.stdin
Of course, I had to escape some problematic characters like the asterisk or semicolon, but there were only a few such characters and they could be done by hand. And I had to create the input feed into stdin but that was easy enough. From there, it's straightforward…ish enough to resolve the issues.
[1] https://github.com/spc476/mod_blog
[2] https://www.gnu.org/software/libc/
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