Segmentation fault at line 756 of ngx_http_ubus_module.c #2
Description
I'm using OpenWRT with nginx and luci_statistics on one of my devices and I am collecting data for quite a lot of other devices.
After adding another device the statistics overview page stopped working and I found messages in the syslog about signal 11 (SIGSEV) for nginx worker processes.
After compiling nginx with debug symbols and installing libc with debug symbols I was able to identify the origin with valgrind in line 756 of ngx_http_ubus_module.c, by the looks because in->buf->pos = 0.
After reading a little bit through the nginx API for ngx_http_read_client_request_body I figured out that this is related to client_body_buffer_size, which defaults on my device to 8192. If the request is bigger (in my case 11520) then it is stored in a temp file rather than in buffers.
From http://nginx.org/en/docs/dev/development_guide.html
"The body can be saved in memory buffers or file buffers, if the capacity specified by the client_body_buffer_size directive is not enough to fit the entire body in memory."
As a workaround I increased client_body_buffer_size to 128k, which solved the problem for me.
Probably it is not worth implementing the file read logic, as usually UBUS requests are pretty small. But I would suggest throwing an error message if in->buf->pos == 0, saying that client_body_buffer_size should be increased.