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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: Nick Desaulniers , Bill Wendling , Justin Stitt , netdev@vger.kernel.org, linux-rdma@vger.kernel.org, patches@lists.linux.dev, llvm@lists.linux.dev References: <20251014-mlx5e-avoid-zero-div-from-mlx5e_mpwrq_umr_entry_size-v1-1-dc186b8819ef@kernel.org> Content-Language: en-US From: Tariq Toukan In-Reply-To: <20251014-mlx5e-avoid-zero-div-from-mlx5e_mpwrq_umr_entry_size-v1-1-dc186b8819ef@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 > --- > 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 > > Thanks for your patch. Reviewed-by: Tariq Toukan