Dumping cyclic error_log memory

Robert Paprocki rpaprocki at fearnothingproductions.net
Sat Jan 4 03:59:58 UTC 2020


http://nginx.org/en/docs/debugging_log.html and associated literature instruct to dump the contents of a “memory” error_log target via gdb memory dump. How is it possible to do this for a given nginx worker process, when other worker processes are serving connections simultaneously and generating error logs to the shared memory zone (at least, I’m assuming it’s shared by all workers, since it’s allocated from the conf pool during configuration parsing). Is it possible other workers are memcpy() with a dst pointing to the buffer where a simultaneous gdb memory dump is executing? Wouldn’t this potentially cause corruption of the read data?

The origin of my question was hacking on a routine that would dump the memory buffer to a file from within nginx directly, rather than probing into the process with gdb- at first it looks trivial to just write() the whole buffer, but since there’s no synchronization mechanism, I anticipate this would cause problems in a busy environment. 

Sent from my iPhone
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