Re: [PATCH net] net/mlx5e: Return 1 instead of 0 in invalid case in mlx5e_mpwrq_umr_entry_size()

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 14/10/2025 23:46, Nathan Chancellor wrote:
> When building with Clang 20 or newer, there are some objtool warnings
> from unexpected fallthroughs to other functions:
> 
>    vmlinux.o: warning: objtool: mlx5e_mpwrq_mtts_per_wqe() falls through to next function mlx5e_mpwrq_max_num_entries()
>    vmlinux.o: warning: objtool: mlx5e_mpwrq_max_log_rq_size() falls through to next function mlx5e_get_linear_rq_headroom()
> 
> LLVM 20 contains an (admittedly problematic [1]) optimization [2] to
> convert divide by zero into the equivalent of __builtin_unreachable(),
> which invokes undefined behavior and destroys code generation when it is
> encountered in a control flow graph.
> 
> mlx5e_mpwrq_umr_entry_size() returns 0 in the default case of an
> unrecognized mlx5e_mpwrq_umr_mode value. mlx5e_mpwrq_mtts_per_wqe(),
> which is inlined into mlx5e_mpwrq_max_log_rq_size(), uses the result of
> mlx5e_mpwrq_umr_entry_size() in a divide operation without checking for
> zero, so LLVM is able to infer there will be a divide by zero in this
> case and invokes undefined behavior. While there is some proposed work
> to isolate this undefined behavior and avoid the destructive code
> generation that results in these objtool warnings, code should still be
> defensive against divide by zero.
> 
> As the WARN_ONCE() implies that an invalid value should be handled
> gracefully, return 1 instead of 0 in the default case so that the
> results of this division operation is always valid.
> 
> Fixes: 168723c1f8d6 ("net/mlx5e: xsk: Use umr_mode to calculate striding RQ parameters")
> Link: https://lore.kernel.org/CAGG=3QUk8-Ak7YKnRziO4=0z=1C_7+4jF+6ZeDQ9yF+kuTOHOQ@mail.gmail.com/ [1]
> Link: https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448 [2]
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2131
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2132
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/en/params.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/params.c b/drivers/net/ethernet/mellanox/mlx5/core/en/params.c
> index 3692298e10f2..c9bdee9a8b30 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/params.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/params.c
> @@ -100,7 +100,7 @@ u8 mlx5e_mpwrq_umr_entry_size(enum mlx5e_mpwrq_umr_mode mode)
>   		return sizeof(struct mlx5_ksm) * 4;
>   	}
>   	WARN_ONCE(1, "MPWRQ UMR mode %d is not known\n", mode);
> -	return 0;
> +	return 1;
>   }
>   
>   u8 mlx5e_mpwrq_log_wqe_sz(struct mlx5_core_dev *mdev, u8 page_shift,
> 
> ---
> base-commit: 4f86eb0a38bc719ba966f155071a6f0594327f34
> change-id: 20251014-mlx5e-avoid-zero-div-from-mlx5e_mpwrq_umr_entry_size-e6d49c18a43f
> 
> Best regards,
> --
> Nathan Chancellor <nathan@kernel.org>
> 
> 

Thanks for your patch.
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>