[PATCH] wifi: mac80211: Replace strncpy() with strscpy_pad() in drv_switch_vif_chanctx tracepoint

[PATCH] wifi: mac80211: Replace strncpy() with strscpy_pad() in drv_switch_vif_chanctx tracepoint

[Kees Cook]

Replace the deprecated[1] strncpy() with strscpy_pad() for copying the
interface name into a tracepoint entry.

The source "sdata->name" is a NUL-terminated char[IFNAMSIZ] buffer
populated via NUL-guaranteeing paths: strscpy() in ieee80211_if_add(),
snprintf() in ieee80211_add_virtual_monitor(), or memcpy() from
ndev->name in ieee80211_if_add() and netdev_notify()
(net/mac80211/iface.c). In the memcpy() cases, the source ndev->name
is itself always NUL-terminated (populated via snprintf() or strscpy()
in __dev_alloc_name() and dev_prep_valid_name() in net/core/dev.c).

The destination "local_vifs[i].vif.vif_name" is a char[IFNAMSIZ] field
in struct trace_vif_entry, stored in a __dynamic_array within the trace
ring buffer. Since ring buffer entries are not zeroed on allocation,
strscpy_pad() is used to zero-fill trailing bytes and prevent exposing
stale ring buffer contents to userspace readers of tracefs.

No behavioral change: since interface names are always at most 15
characters plus a NUL terminator, strscpy_pad() with size IFNAMSIZ (16)
produces identical output to the original strncpy().

Link: https://github.com/KSPP/linux/issues/90 [1]
Signed-off-by: Kees Cook <kees@kernel.org>
---
 net/mac80211/trace.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/mac80211/trace.h b/net/mac80211/trace.h
index c04d4547e8f4..3b139fb67d1b 100644
--- a/net/mac80211/trace.h
+++ b/net/mac80211/trace.h
@@ -1796,9 +1796,8 @@ TRACE_EVENT(drv_switch_vif_chanctx,
 				SWITCH_ENTRY_ASSIGN(vif.vif_type, vif->type);
 				SWITCH_ENTRY_ASSIGN(vif.p2p, vif->p2p);
 				SWITCH_ENTRY_ASSIGN(link_id, link_conf->link_id);
-				strncpy(local_vifs[i].vif.vif_name,
-					sdata->name,
-					sizeof(local_vifs[i].vif.vif_name));
+				strscpy_pad(local_vifs[i].vif.vif_name,
+					    sdata->name);
 				SWITCH_ENTRY_ASSIGN(old_chandef.control_freq,
 						old_ctx->def.chan->center_freq);
 				SWITCH_ENTRY_ASSIGN(old_chandef.freq_offset,
-- 
2.34.1

Re: [PATCH] wifi: mac80211: Replace strncpy() with strscpy_pad() in drv_switch_vif_chanctx tracepoint

[Johannes Berg]

On Mon, 2026-03-23 at 10:20 -0700, Kees Cook wrote:
> ring buffer. Since ring buffer entries are not zeroed on allocation,
> 
> No behavioral change: since interface names are always at most 15
> characters plus a NUL terminator, strscpy_pad() with size IFNAMSIZ (16)
> produces identical output to the original strncpy().

This doesn't add up - if it's not zero-filled on allocation, then it
*is* a (desired) behaviour change?

johannes

Re: [PATCH] wifi: mac80211: Replace strncpy() with strscpy_pad() in drv_switch_vif_chanctx tracepoint

[Johannes Berg]

On Mon, 2026-03-23 at 18:32 +0100, Johannes Berg wrote:
> On Mon, 2026-03-23 at 10:20 -0700, Kees Cook wrote:
> > ring buffer. Since ring buffer entries are not zeroed on allocation,
> > 
> > No behavioral change: since interface names are always at most 15
> > characters plus a NUL terminator, strscpy_pad() with size IFNAMSIZ (16)
> > produces identical output to the original strncpy().
> 
> This doesn't add up - if it's not zero-filled on allocation, then it
> *is* a (desired) behaviour change?

Err, no, never mind, it's str*n*cpy() now...

johannes