Re: [PATCH bpf-next v4 0/3] bpf: Report arena faults to BPF streams

[Puranjay Mohan <puranjay@kernel.org>]

Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> On Wed, 27 Aug 2025 at 17:37, Puranjay Mohan <puranjay@kernel.org> wrote:
>>
>> Changes in v3->v4:
>> v3: https://lore.kernel.org/all/20250827150113.15763-1-puranjay@kernel.org/
>> - Fixed a build issue when CONFIG_BPF_JIT=y and # CONFIG_BPF_SYSCALL is not set
>>
>> Changes in v2->v3:
>> v2: https://lore.kernel.org/all/20250811111828.13836-1-puranjay@kernel.org/
>> - Improved the selftest to check the exact fault address
>> - Dropped BPF_NO_KFUNC_PROTOTYPES and bpf_arena_alloc/free_pages() usage
>> - Rebased on bpf-next/master
>>
>> Changes in v1->v2:
>> v1: https://lore.kernel.org/all/20250806085847.18633-1-puranjay@kernel.org/
>> - Changed variable and mask names for consistency (Yonghong)
>> - Added Acked-by: Yonghong Song <yonghong.song@linux.dev> on two patches
>>
>> This set adds the support of reporting page faults inside arena to BPF
>> stderr stream. The reported address is the one that a user would expect
>> to see if they pass it to bpf_printk();
>>
>> Here is an example output from a stream and bpf_printk()
>>
>> ERROR: Arena WRITE access at unmapped address 0xdeaddead0000
>> CPU: 9 UID: 0 PID: 502 Comm: test_progs
>> Call trace:
>> bpf_stream_stage_dump_stack+0xc0/0x150
>> bpf_prog_report_arena_violation+0x98/0xf0
>> ex_handler_bpf+0x5c/0x78
>> fixup_exception+0xf8/0x160
>> __do_kernel_fault+0x40/0x188
>> do_bad_area+0x70/0x88
>> do_translation_fault+0x54/0x98
>> do_mem_abort+0x4c/0xa8
>> el1_abort+0x44/0x70
>> el1h_64_sync_handler+0x50/0x108
>> el1h_64_sync+0x6c/0x70
>> bpf_prog_a64a9778d31b8e88_stream_arena_write_fault+0x84/0xc8
>>   *(page) = 1; @ stream.c:100
>> bpf_prog_test_run_syscall+0x100/0x328
>> __sys_bpf+0x508/0xb98
>> __arm64_sys_bpf+0x2c/0x48
>> invoke_syscall+0x50/0x120
>> el0_svc_common.constprop.0+0x48/0xf8
>> do_el0_svc+0x28/0x40
>> el0_svc+0x48/0xf8
>> el0t_64_sync_handler+0xa0/0xe8
>> el0t_64_sync+0x198/0x1a0
>>
>> Same address is seen by using bpf_printk():
>>
>> 1389.078831: bpf_trace_printk: Read Address: 0xdeaddead0000
>>
>> To make this possible, some extra metadata has to be passed to the bpf
>> exception handler, so the bpf exception handling mechanism for both
>> x86-64 and arm64 have been improved in this set.
>>
>> The streams selftest has been updated to also test this new feature.
>
> We also need arm64 experts to take a look before we land, since you'll
> respin anyway now.
> Xu, could you please provide acks on the patches?
>
> Thanks a lot.

Thanks for your review.
I will wait for Xu's feedback before respining.

Thanks,
Puranjay