PCI/P2PDMA: Safely terminate ACS redirect lists

seq_buf marks an overflow by setting len to size + 1. The ACS diagnostic
path unconditionally writes a terminator to buffer[len - 1], so a path with
enough ACS ports to fill the 128-byte buffer writes one byte beyond the
buffer when verbose diagnostics are requested.

Use seq_buf_str() to terminate truncated output safely and remove the final
semicolon only when the buffer did not overflow.

Fixes: 52916982af48 ("PCI/P2PDMA: Support peer-to-peer memory")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Tushar Dave <tdave@nvidia.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://patch.msgid.link/20260830-batch-p2p-fixes-v1-4-5044e8dfbe2e@nvidia.com

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
index 509813d75467..bf5af4555f99 100644
--- a/drivers/pci/p2pdma.c
+++ b/drivers/pci/p2pdma.c
@@ -778,11 +778,13 @@ check_b_path_acs:
 	}
 
 	if (verbose) {
-		acs_list.buffer[acs_list.len-1] = 0; /* drop final semicolon */
+		/* Drop the final semicolon; the list is not empty here */
+		if (!seq_buf_has_overflowed(&acs_list))
+			acs_list.buffer[acs_list.len - 1] = '\0';
 		pci_warn(client, "ACS redirect is set between the client and provider (%s)\n",
 			 pci_name(provider));
 		pci_warn(client, "to disable ACS redirect for this path, add the kernel parameter: pci=disable_acs_redir=%s\n",
-			 acs_list.buffer);
+			 seq_buf_str(&acs_list));
 	}
 	acs_redirects = true;