"Naveen N. Rao" [off-list ref] writes:

Michael Ellerman wrote:

quoted

Daniel Borkmann [off-list ref] writes:

quoted

On 10/25/21 8:15 AM, Naveen N. Rao wrote:

quoted

Hari Bathini wrote:

quoted

Running program with bpf-to-bpf function calls results in data access
exception (0x300) with the below call trace:

    [c000000000113f28] bpf_int_jit_compile+0x238/0x750 (unreliable)
    [c00000000037d2f8] bpf_check+0x2008/0x2710
    [c000000000360050] bpf_prog_load+0xb00/0x13a0
    [c000000000361d94] __sys_bpf+0x6f4/0x27c0
    [c000000000363f0c] sys_bpf+0x2c/0x40
    [c000000000032434] system_call_exception+0x164/0x330
    [c00000000000c1e8] system_call_vectored_common+0xe8/0x278

as bpf_int_jit_compile() tries writing to write protected JIT code
location during the extra pass.

Fix it by holding off write protection of JIT code until the extra
pass, where branch target addresses fixup happens.

Cc: stable@vger.kernel.org
Fixes: 62e3d4210ac9 ("powerpc/bpf: Write protect JIT code")
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
---
 arch/powerpc/net/bpf_jit_comp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Thanks for the fix!

Reviewed-by: Naveen N. Rao <redacted>

LGTM, I presume this fix will be routed via Michael.

Thanks for reviewing, I've picked it up.

quoted

BPF selftests have plenty of BPF-to-BPF calls in there, too bad this was
caught so late. :/

Yeah :/

STRICT_KERNEL_RWX is not on by default in all our defconfigs, so that's
probably why no one caught it.

Yeah, sorry - we should have caught this sooner.

quoted

I used to run the BPF selftests but they stopped building for me a while
back, I'll see if I can get them going again.

Ravi had started looking into getting the selftests working well before 
he left. I will take a look at this.

Thanks.

I got them building with something like:

 - turning on DEBUG_INFO and DEBUG_INFO_BTF and rebuilding vmlinux
 - grabbing clang 13 from: 
   https://github.com/llvm/llvm-project/releases/download/llvmorg-13.0.0/clang+llvm-13.0.0-powerpc64le-linux-ubuntu-18.04.tar.xz
 - PATH=$HOME/clang+llvm-13.0.0-powerpc64le-linux-ubuntu-18.04/bin/:$PATH
 - apt install:
   - libelf-dev
   - dwarves
   - python-docutils
   - libcap-dev


The DEBUG_INFO requirement is a bit of a pain for me. I generally don't
build with that enabled, because the resulting kernels are stupidly
large. I'm not sure if that's a hard requirement, or if the vmlinux has
to match the running kernel exactly?

There is logic in tools/testing/bpf/Makefile to use VMLINUX_H instead of
extracting the BTF from the vmlinux (line 247), but AFAICS that's
unreachable since 1a3449c19407 ("selftests/bpf: Clarify build error if
no vmlinux"), which makes it a hard error to not have a VMLINUX_BTF.

cheers