Re: [PATCH bpf] bpf, arm64: Fix bpf trampoline instruction endianness

From: Daniel Borkmann
Date: Wed Aug 10 2022 - 10:54:15 EST


On 8/9/22 12:33 PM, Jean-Philippe Brucker wrote:
[+ arm64 maintainers]

On Mon, Aug 08, 2022 at 12:07:35AM -0400, Xu Kuohai wrote:
The sparse tool complains as follows:

arch/arm64/net/bpf_jit_comp.c:1684:16:
warning: incorrect type in assignment (different base types)
arch/arm64/net/bpf_jit_comp.c:1684:16:
expected unsigned int [usertype] *branch
arch/arm64/net/bpf_jit_comp.c:1684:16:
got restricted __le32 [usertype] *
arch/arm64/net/bpf_jit_comp.c:1700:52:
error: subtraction of different types can't work (different base
types)
arch/arm64/net/bpf_jit_comp.c:1734:29:
warning: incorrect type in assignment (different base types)
arch/arm64/net/bpf_jit_comp.c:1734:29:
expected unsigned int [usertype] *
arch/arm64/net/bpf_jit_comp.c:1734:29:
got restricted __le32 [usertype] *
arch/arm64/net/bpf_jit_comp.c:1918:52:
error: subtraction of different types can't work (different base
types)

This is because the variable branch in function invoke_bpf_prog and the
variable branches in function prepare_trampoline are defined as type
u32 *, which conflicts with ctx->image's type __le32 *, so sparse complains
when assignment or arithmetic operation are performed on these two
variables and ctx->image.

Since arm64 instructions are always little-endian, change the type of
these two variables to __le32 * and call cpu_to_le32 to convert
instruction to little-endian before writing it to memory.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Fixes: efc9909fdce0 ("bpf, arm64: Add bpf trampoline for arm64")
Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>

Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>

Applied, thanks! Also added small note that this is in line with emit() as well.