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

[Nathan Chancellor <nathan@kernel.org>]

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>