Oh my god, same. This literally catches bugs with a smoking gun in their hand in a way that's completely impossible with printf. I'd upvote this 100 times if I could.

▲

sherincall 5 days ago | parent [-]

> completely impossible with printf

Not printf exactly, but I've found bugs with a combination of mprotect, userfaultfd and backtrace_symbols when I couldn't use HW breakpoints.

Basically, mark a set of pages as non-writable so that any writes trigger a pagefault, then register yourself as a pagefault handler for those and see who is doing the write, apply the write and move on. You can do this with LD_PRELOAD without even recompiling the debugee.

	▲	antonchekhov 4 days ago \| parent \| next [-]
		Back in the early '90s, I was a big fan of the C/C++ debugging library "electric fence" (written by Bruce Perens) - it was a malloc() implementation that used mmap() to set the pages of the returned buffer such that any writes in that region (and even read accesses!) caused a segfault to happen, and the program halts, so you can examine the stack. It was a godsend.
	▲	jesse__ 4 days ago \| parent \| prev [-]
		Oh yeah, I've done that trick before, it's quite handy. Sometimes catches bugs that would be tricky to find with watchpoints. I wrote an allocator that allocates an extra page either before or after the users allocation, and aligns the user allocation such that it is as close as possible (while respecting user-requested alignment) to butting up against that extra page (marked PROT_NONE). Can catch underflows or overflows with this method, but not both simultaneously. Very handy from time to time.