Re: [PATCH net 3/3] octeontx2-af: npc: cn20k: Return error when defrag rollback free fails

[Simon Horman <horms@kernel.org>]

From: 'Simon Horman' <horms@kernel.org>

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octeontx2-af: npc: cn20k: Return error when defrag rollback free fails

This commit attempts to fix a bug where an error code wasn't returned
when a rollback failure occurred during partial allocation in
npc_defrag_alloc_free_slots(). It sets rc to -EFAULT if the
__npc_subbank_free() call fails.

Link: https://lore.kernel.org/netdev/adjNJEpILRZATB2N@stanley.mountain/

> diff --git a/drivers/net/ethernet/marvell/octeontx2/af/cn20k/npc.c b/drivers/net/ethernet/marvell/octeontx2/af/cn20k/npc.c
> index 2fcd0ee2b1e10..df192729ac1d4 100644
> --- a/drivers/net/ethernet/marvell/octeontx2/af/cn20k/npc.c
> +++ b/drivers/net/ethernet/marvell/octeontx2/af/cn20k/npc.c
> @@ -3541,6 +3541,7 @@ static int npc_defrag_alloc_free_slots(struct rvu *rvu,
>  			dev_err(rvu->dev,
>  				"%s: Error to free mcam idx=%u\n",
>  				__func__, save[i]);
> +			rc = -EFAULT;
>  			break;
>  		}
>  	}

While this ensures an error is returned if the rollback fails, does the
function still incorrectly return success when the rollback succeeds?

Earlier in the function, if the allocation cannot fulfill the requested slots,
the code jumps to the rollback loop:

	if (alloc_cnt1 + alloc_cnt2 != cnt) {
		...
		goto fail_free_alloc;
	}

Inside the fail_free_alloc loop, the rc variable is overwritten by the
subbank lookup:

fail_free_alloc:
	for (i = 0; i < alloc_cnt1 + alloc_cnt2; i++) {
		rc = npc_mcam_idx_2_subbank_idx(rvu, save[i], &sb, &sb_off);

If the subbank lookup and the subsequent __npc_subbank_free() succeed,
rc will be 0.

Since the loop finishes normally without hitting the newly added rc = -EFAULT
path, does the function end up returning 0 at the end of the fail_free_alloc
block?

Could this cause the caller to incorrectly assume that all slots were
successfully allocated, leading to operations on the freed or uninitialized
slots in the save array? Using a separate variable for the loop operations
or explicitly setting an error code like -ENOSPC before the goto might help
preserve the failure state.