Re: [PATCH v2] powerpc/bpf/32: Fix Oops on tail call tests

[Naveen N. Rao]

Christophe Leroy wrote:
> test_bpf tail call tests end up as:
>
>   test_bpf: #0 Tail call leaf jited:1 85 PASS
>   test_bpf: #1 Tail call 2 jited:1 111 PASS
>   test_bpf: #2 Tail call 3 jited:1 145 PASS
>   test_bpf: #3 Tail call 4 jited:1 170 PASS
>   test_bpf: #4 Tail call load/store leaf jited:1 190 PASS
>   test_bpf: #5 Tail call load/store jited:1
>   BUG: Unable to handle kernel data access on write at 0xf1b4e000
>   Faulting instruction address: 0xbe86b710
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   BE PAGE_SIZE=4K MMU=Hash PowerMac
>   Modules linked in: test_bpf(+)
>   CPU: 0 PID: 97 Comm: insmod Not tainted 6.1.0-rc4+ #195
>   Hardware name: PowerMac3,1 750CL 0x87210 PowerMac
>   NIP:  be86b710 LR: be857e88 CTR: be86b704
>   REGS: f1b4df20 TRAP: 0300   Not tainted  (6.1.0-rc4+)
>   MSR:  00009032 <EE,ME,IR,DR,RI>  CR: 28008242  XER: 00000000
>   DAR: f1b4e000 DSISR: 42000000
>   GPR00: 00000001 f1b4dfe0 c11d2280 00000000 00000000 00000000 00000002 00000000
>   GPR08: f1b4e000 be86b704 f1b4e000 00000000 00000000 100d816a f2440000 fe73baa8
>   GPR16: f2458000 00000000 c1941ae4 f1fe2248 00000045 c0de0000 f2458030 00000000
>   GPR24: 000003e8 0000000f f2458000 f1b4dc90 3e584b46 00000000 f24466a0 c1941a00
>   NIP [be86b710] 0xbe86b710
>   LR [be857e88] __run_one+0xec/0x264 [test_bpf]
>   Call Trace:
>   [f1b4dfe0] [00000002] 0x2 (unreliable)
>   Instruction dump:
>   XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>   XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>   ---[ end trace 0000000000000000 ]---
>
> This is a tentative to write above the stack. The problem is encoutered
> with tests added by commit 38608ee7b690 ("bpf, tests: Add load store
> test case for tail call")
>
> This happens because tail call is done to a BPF prog with a different
> stack_depth. At the time being, the stack is kept as is when the caller
> tail calls its callee. But at exit, the callee restores the stack based
> on its own properties. Therefore here, at each run, r1 is erroneously
> increased by 32 - 16 = 16 bytes.
>
> This was done that way in order to pass the tail call count from caller
> to callee through the stack. As powerpc32 doesn't have a red zone in
> the stack, it was necessary the maintain the stack as is for the tail
> call. But it was not anticipated that the BPF frame size could be
> different.
>
> Let's take a new approach. Use register r4 to carry the tail call count
> during the tail call, and save it into the stack at function entry if
> required. This means the input parameter must be in r3, which is more
> correct as it is a 32 bits parameter, then tail call better match with
> normal BPF function entry, the down side being that we move that input
> parameter back and forth between r3 and r4. That can be optimised later.
>
> Doing that also has the advantage of maximising the common parts between
> tail calls and a normal function exit.
>
> With the fix, tail call tests are now successfull:
>
>   test_bpf: #0 Tail call leaf jited:1 53 PASS
>   test_bpf: #1 Tail call 2 jited:1 115 PASS
>   test_bpf: #2 Tail call 3 jited:1 154 PASS
>   test_bpf: #3 Tail call 4 jited:1 165 PASS
>   test_bpf: #4 Tail call load/store leaf jited:1 101 PASS
>   test_bpf: #5 Tail call load/store jited:1 141 PASS
>   test_bpf: #6 Tail call error path, max count reached jited:1 994 PASS
>   test_bpf: #7 Tail call count preserved across function calls jited:1 140975 PASS
>   test_bpf: #8 Tail call error path, NULL target jited:1 110 PASS
>   test_bpf: #9 Tail call error path, index out of range jited:1 69 PASS
>   test_bpf: test_tail_calls: Summary: 10 PASSED, 0 FAILED, [10/10 JIT'ed]
>
> Suggested-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> Fixes: 51c66ad849a7 ("powerpc/bpf: Implement extended BPF on PPC32")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>

Tested-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx


Thanks,
Naveen