[tip: x86/cache] fs/resctrl: Inform user space when status buffer overflowed

[tip-bot2 for Reinette Chatre]

The following commit has been merged into the x86/cache branch of tip:

Commit-ID:     854db793d4073087fa799d886ed84a342e3f484a
Gitweb:        https://git.kernel.org/tip/854db793d4073087fa799d886ed84a342e3f484a
Author:        Reinette Chatre <reinette.chatre@xxxxxxxxx>
AuthorDate:    Tue, 30 Jun 2026 21:27:10 -07:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Sun, 26 Jul 2026 21:40:26 -07:00

fs/resctrl: Inform user space when status buffer overflowed

resctrl fs commands are becoming more powerful with, for example, a user able
to use syntax like '*' to make broad configuration changes. Such commands that
span multiple domains may result in more than one message printed to the
last_cmd_status buffer with more planned.

Issue "[truncated]" when displaying the last_cmd_status buffer to communicate
if it overflowed. Upon encountering this, user space is expected to combine
details about the failure found in info/last_cmd_status with related resctrl
files to learn the accurate system state after the command failure.

Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Ben Horgan <ben.horgan@xxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Tested-by: Babu Moger <babu.moger@xxxxxxx>
Link: https://patch.msgid.link/eb3d1aaa41d1f2bf6b1bb26ceafd52f04912121d.1782857711.git.reinette.chatre@xxxxxxxxx
---
 fs/resctrl/rdtgroup.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 08d58c4..5dcbb0a 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -979,10 +979,13 @@ static int rdt_last_cmd_status_show(struct kernfs_open_file *of,
 	if (!info_kn_lock(of->kn))
 		return -ENOENT;
 	len = seq_buf_used(&last_cmd_status);
-	if (len)
+	if (len) {
 		seq_printf(seq, "%.*s", len, last_cmd_status_buf);
-	else
+		if (seq_buf_has_overflowed(&last_cmd_status))
+			seq_puts(seq, "[truncated]\n");
+	} else {
 		seq_puts(seq, "ok\n");
+	}
 	info_kn_unlock(of->kn);
 	return 0;
 }