[PATCH] x86/intel_rdt: Fix a silent failure when writing zero value schemata

[Tony Luck <tony.luck@intel.com>]

From: Xiaochen Shen <xiaochen.shen@intel.com>

When writing an invalid schemata with no domain values (e.g., "(L3|MB):"), we
get a silent failure.

There isn't a good reason to allow writing zero value, we'd better
return error and set diagnostics information in last_cmd_status.

1. Before the fix:
 # mkdir /sys/fs/resctrl/p1

 # echo "L3:" > /sys/fs/resctrl/p1/schemata
 (silent failure)
 # cat /sys/fs/resctrl/info/last_cmd_status
 ok

 # echo "MB:" > /sys/fs/resctrl/p1/schemata
 (silent failure)
 # cat /sys/fs/resctrl/info/last_cmd_status
 ok

2. After the fix:
 # mkdir /sys/fs/resctrl/p1

 # echo "L3:" > /sys/fs/resctrl/p1/schemata
 -bash: echo: write error: Invalid argument
 # cat /sys/fs/resctrl/info/last_cmd_status
 Missing 'L3' value

 # echo "MB:" > /sys/fs/resctrl/p1/schemata
 -bash: echo: write error: Invalid argument
 # cat /sys/fs/resctrl/info/last_cmd_status
 Missing 'MB' value

[ Tony: This is an unintended side effect of the patch earlier to allow
    the user to just write the value they want to change.
    While allowing user to specify less than all of the values, it also
    allows zero value. ]

Fixes: c4026b7b95a4 ("x86/intel_rdt: Implement "update" mode when writing schemata file")
Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
---
[apply to TIP x86/cache]

 arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c b/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
index 30aeb267cbd2..23e1d5c249c6 100644
--- a/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
+++ b/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
@@ -257,6 +257,11 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
 			ret = -EINVAL;
 			goto out;
 		}
+		if (tok[0] == '\0') {
+			rdt_last_cmd_printf("Missing '%s' value\n", resname);
+			ret = -EINVAL;
+			goto out;
+		}
 		ret = rdtgroup_parse_resource(resname, tok, closid);
 		if (ret)
 			goto out;
-- 
2.14.1