When the inferior is stopped inside a syscall (e.g. poll), single-stepping out of this syscall (via "step") causes GDB to loop and to slowly consume more and more memory.  This can easily be seen when debugging GDB with GDB:

$ gdb gdb
(gdb) run
Starting program: /scratchpad/binutils-gdb-build/gdb/gdb 
(gdb) ^C
Program received signal SIGINT, Interrupt.
0x00007ffff6d784f0 in poll () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) step 
Single stepping until exit from function poll,
which has no line number information.
<infinite loop here>