Re: [PATCH] bpf: selftest for late caller stack size increase

On Fri, Dec 22, 2017 at 07:12:35PM +0100, Jann Horn wrote:

This checks that it is not possible to bypass the total stack size check in
update_stack_depth() by calling a function that uses a large amount of
stack memory *before* using a large amount of stack memory in the caller.

Currently, the first added testcase causes a rejection as expected, but
the second testcase is (AFAICS incorrectly) accepted:

[...]
#483/p calls: stack overflow using two frames (post-call access) FAIL
Unexpected success to load!
0: (85) call pc+2
caller:
 R10=fp0,call_-1
callee:
 frame1: R1=ctx(id=0,off=0,imm=0) R10=fp0,call_0
3: (72) *(u8 *)(r10 -300) = 0
4: (b7) r0 = 0
5: (95) exit
returning from callee:
 frame1: R0_w=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0,call_0
to caller at 1:
 R0_w=inv0 R10=fp0,call_-1

from 5 to 1: R0=inv0 R10=fp0,call_-1
1: (72) *(u8 *)(r10 -300) = 0
2: (95) exit
processed 6 insns, stack depth 300+300

got it. thanks for the test!
working on a fix.