[PATCH v2 net-next 2/4] net: add skeleton of bpfilter kernel module

On Mon, May 07, 2018 at 06:51:24PM +0000, Luis R. Rodriguez wrote:

quoted

Notice that _binary_net_bpfilter_bpfilter_umh_start - end
is placed into .init.rodata section, so it's freed as soon as __init
function of bpfilter.ko is finished.
As part of __init the bpfilter.ko does first request/reply action
via two unix pipe provided by fork_usermode_blob() helper to
make sure that umh is healthy. If not it will kill it via pid.

It does this very fast, right away. On a really slow system how are you sure
that this won't race and the execution of the check happens early on prior to
letting the actual setup trigger? After all, we're calling the userpsace
process in async mode. We could preempt it now.

I don't see an issue.
the kernel synchronously writes into a pipe. User space process reads.
Exactly the same as coredump logic with pipes.

quoted

+# a bit of elf magic to convert bpfilter_umh binary into a binary blob
+# inside bpfilter_umh.o elf file referenced by
+# _binary_net_bpfilter_bpfilter_umh_start symbol
+# which bpfilter_kern.c passes further into umh blob loader at run-time
+quiet_cmd_copy_umh = GEN $@
+      cmd_copy_umh = echo ':' > $(obj)/.bpfilter_umh.o.cmd; \
+      $(OBJCOPY) -I binary -O $(CONFIG_OUTPUT_FORMAT) \
+      -B `$(OBJDUMP) -f $<|grep architecture|cut -d, -f1|cut -d' ' -f2` \
+      --rename-section .data=.init.rodata $< $@

Cool, but so our expectation is that the compiler sets this symbol, how
are we sure it will always be set?

Compiler doesn't set it. objcopy does.

quoted

+
+	if (__bpfilter_process_sockopt(NULL, 0, 0, 0, 0) != 0) {

See, here, what if the userspace process gets preemtped and we run this
check afterwards? Is that possible?

User space is a normal task. It can sleep and can be single stepped with GDB.

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