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

[Jean-Philippe Brucker]

[+ 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>

> ---
>  arch/arm64/net/bpf_jit_comp.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 7ca8779ae34f..29dc55da2476 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -1643,7 +1643,7 @@ static void invoke_bpf_prog(struct jit_ctx *ctx, struct bpf_tramp_link *l,
>  			    int args_off, int retval_off, int run_ctx_off,
>  			    bool save_ret)
>  {
> -	u32 *branch;
> +	__le32 *branch;
>  	u64 enter_prog;
>  	u64 exit_prog;
>  	struct bpf_prog *p = l->link.prog;
> @@ -1698,7 +1698,7 @@ static void invoke_bpf_prog(struct jit_ctx *ctx, struct bpf_tramp_link *l,
>  
>  	if (ctx->image) {
>  		int offset = &ctx->image[ctx->idx] - branch;
> -		*branch = A64_CBZ(1, A64_R(0), offset);
> +		*branch = cpu_to_le32(A64_CBZ(1, A64_R(0), offset));
>  	}
>  
>  	/* arg1: prog */
> @@ -1713,7 +1713,7 @@ static void invoke_bpf_prog(struct jit_ctx *ctx, struct bpf_tramp_link *l,
>  
>  static void invoke_bpf_mod_ret(struct jit_ctx *ctx, struct bpf_tramp_links *tl,
>  			       int args_off, int retval_off, int run_ctx_off,
> -			       u32 **branches)
> +			       __le32 **branches)
>  {
>  	int i;
>  
> @@ -1784,7 +1784,7 @@ static int prepare_trampoline(struct jit_ctx *ctx, struct bpf_tramp_image *im,
>  	struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
>  	struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
>  	bool save_ret;
> -	u32 **branches = NULL;
> +	__le32 **branches = NULL;
>  
>  	/* trampoline stack layout:
>  	 *                  [ parent ip         ]
> @@ -1892,7 +1892,7 @@ static int prepare_trampoline(struct jit_ctx *ctx, struct bpf_tramp_image *im,
>  				flags & BPF_TRAMP_F_RET_FENTRY_RET);
>  
>  	if (fmod_ret->nr_links) {
> -		branches = kcalloc(fmod_ret->nr_links, sizeof(u32 *),
> +		branches = kcalloc(fmod_ret->nr_links, sizeof(__le32 *),
>  				   GFP_KERNEL);
>  		if (!branches)
>  			return -ENOMEM;
> @@ -1916,7 +1916,7 @@ static int prepare_trampoline(struct jit_ctx *ctx, struct bpf_tramp_image *im,
>  	/* update the branches saved in invoke_bpf_mod_ret with cbnz */
>  	for (i = 0; i < fmod_ret->nr_links && ctx->image != NULL; i++) {
>  		int offset = &ctx->image[ctx->idx] - branches[i];
> -		*branches[i] = A64_CBNZ(1, A64_R(10), offset);
> +		*branches[i] = cpu_to_le32(A64_CBNZ(1, A64_R(10), offset));
>  	}
>  
>  	for (i = 0; i < fexit->nr_links; i++)
> -- 
> 2.30.2
>