Re: [PATCH bpf-next] bpf, riscv: Fix stack layout of JITed code on RV32

[Xi Wang]

On Wed, Apr 29, 2020 at 5:51 PM Luke Nelson <lukenels@xxxxxxxxxxxxxxxxx> wrote:
>
> This patch fixes issues with stackframe unwinding and alignment in the
> current stack layout for BPF programs on RV32.
>
> In the current layout, RV32 fp points to the JIT scratch registers, rather
> than to the callee-saved registers. This breaks stackframe unwinding,
> which expects fp to point just above the saved ra and fp registers.
>
> This patch fixes the issue by moving the callee-saved registers to be
> stored on the top of the stack, pointed to by fp. This satisfies the
> assumptions of stackframe unwinding.
>
> This patch also fixes an issue with the old layout that the stack was
> not aligned to 16 bytes.
>
> Stacktrace from JITed code using the old stack layout:
>
>   [   12.196249 ] [<c0402200>] walk_stackframe+0x0/0x96
>
> Stacktrace using the new stack layout:
>
>   [   13.062888 ] [<c0402200>] walk_stackframe+0x0/0x96
>   [   13.063028 ] [<c04023c6>] show_stack+0x28/0x32
>   [   13.063253 ] [<a403e778>] bpf_prog_82b916b2dfa00464+0x80/0x908
>   [   13.063417 ] [<c09270b2>] bpf_test_run+0x124/0x39a
>   [   13.063553 ] [<c09276c0>] bpf_prog_test_run_skb+0x234/0x448
>   [   13.063704 ] [<c048510e>] __do_sys_bpf+0x766/0x13b4
>   [   13.063840 ] [<c0485d82>] sys_bpf+0xc/0x14
>   [   13.063961 ] [<c04010f0>] ret_from_syscall+0x0/0x2
>
> The new code is also simpler to understand and includes an ASCII diagram
> of the stack layout.
>
> Tested on riscv32 QEMU virt machine.
>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>

Thanks for the fix!

Acked-by: Xi Wang <xi.wang@xxxxxxxxx>