Memory tracking miscount

[gbour]

Hello,

We hit a memory tracking issue in mosquitto (both 1.6.4 and master tested). This leads to client connections being rejected when used memory exceed the limit set in mosquitto.conf (memory tracking enabled).
We identified that at some point, there is a memcount counter underflow and because it is a unsigned long, its value becomes greater than max_memcount and all new client connections are rejected.
The role of our mosquitto-memleak.patch patch is to print a stacktrace and force-crashing mosquitto when the underflow happens.

100 void mosquitto__free(void *mem)                                                                      
101 {                                                                                                    
102 #ifdef REAL_WITH_MEMORY_TRACKING                                                                     
103     if(!mem){                                                                                        
104         return;                                                                                      
105     }                                                                                                
106     if (memcount < malloc_usable_size(mem)) {                                                        
107         print_stacktrace();                                                                          
108         assert (0);                                                                                  
109     } 

This is 100% reproducible with following scenario:

shell1> patch -p1 < mosquitto-memleak.patch
shell1> ./src/mosquitto -c mosquitto-memleak.conf
shell2> while true; do mosquitto_sub -h 127.0.0.1 -t '$PROBE' -E -d; sleep .1; done

in less than 10 minutes, mosquitto is crashing with following stacktrace:

=== BACKTRACE ===
./src/mosquitto(mosquitto__free+0x48)[0x4162d8]
./src/mosquitto(context__free_disused+0x37)[0x40e9a7]
./src/mosquitto(mosquitto_main_loop+0x1d6)[0x415756]
./src/mosquitto(main+0x338)[0x409148]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb)[0x7f91695ecbbb]
./src/mosquitto(_start+0x2a)[0x4092ca]
=== /BACKTRACE ===

mosquitto-memleak.zip contains:

• mosquitto-memleak.patch
• mosquitto-memleak.conf
• mosquitto-memleak.core

[ralight]

Thanks for the report - as you've just noted it's a memory tracking problem not a leak. I've just pushed a fix.

[gbour]

You're awesome 👍
I'll test it

[gbour]

works perfectly fine, I cannot hit the issue anymore

[ralight]

Thank you for confirming.