[PATCH AUTOSEL 6.16-6.12] tools/power turbostat: Handle cap_get_proc() ENOSYS

[Sasha Levin <sashal@kernel.org>]

From: Calvin Owens <calvin@wbinvd.org>

[ Upstream commit d34fe509f5f76d9dc36291242d67c6528027ebbd ]

Kernels configured with CONFIG_MULTIUSER=n have no cap_get_proc().
Check for ENOSYS to recognize this case, and continue on to
attempt to access the requested MSRs (such as temperature).

Signed-off-by: Calvin Owens <calvin@wbinvd.org>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

**Backport Status: YES**

This commit should be backported to stable kernel trees for the
following reasons:

## Bug Fix Nature
The commit fixes a clear bug where turbostat fails to work on kernels
configured with `CONFIG_MULTIUSER=n`. Without this fix, turbostat would
incorrectly fail when `cap_get_proc()` returns NULL with `ENOSYS`,
preventing users from accessing MSRs (like temperature readings) even
when they should be able to.

## Small and Contained Change
The fix is minimal - only 9 lines added, 1 line removed. The change is
entirely contained within the `check_for_cap_sys_rawio()` function in
turbostat.c:
- It adds a check for `errno == ENOSYS` when `cap_get_proc()` returns
  NULL
- Returns 0 (success) in this specific case to allow MSR access attempts
  to proceed
- Preserves the original error handling (return 1) for all other failure
  cases

## No Side Effects or Architectural Changes
The change doesn't introduce any new features or architectural
modifications. It simply adds proper error handling for a specific
configuration scenario that was previously not handled correctly. The
logic flow remains the same for all other cases - only the ENOSYS case
gets special treatment.

## User Impact
This affects real users running embedded or specialized Linux systems
with `CONFIG_MULTIUSER=n` (single-user mode kernels). Without this fix,
turbostat is completely broken on such systems, unable to read MSRs for
temperature monitoring and other power management features.

## Stable Tree Criteria Compliance
The fix meets stable kernel criteria:
- **Fixes a real bug**: turbostat failure on CONFIG_MULTIUSER=n kernels
- **Already upstream**: The commit is in the mainline kernel
- **Minimal risk**: The change only affects the specific error case and
  doesn't alter behavior for normal configurations
- **Clear and obvious**: The fix is straightforward - checking for
  ENOSYS and handling it appropriately

## Tool-Specific Nature
Since this is a userspace tool fix (tools/power/x86/turbostat/), it has
even lower risk of causing kernel regressions while providing immediate
benefit to affected users.

The commit is an ideal candidate for stable backporting as it fixes a
specific, reproducible issue with minimal code changes and no risk to
systems where `CONFIG_MULTIUSER=y` (the common case).

 tools/power/x86/turbostat/turbostat.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/power/x86/turbostat/turbostat.c b/tools/power/x86/turbostat/turbostat.c
index 46ee85216373..00fdb6589bea 100644
--- a/tools/power/x86/turbostat/turbostat.c
+++ b/tools/power/x86/turbostat/turbostat.c
@@ -6573,8 +6573,16 @@ int check_for_cap_sys_rawio(void)
 	int ret = 0;
 
 	caps = cap_get_proc();
-	if (caps == NULL)
+	if (caps == NULL) {
+		/*
+		 * CONFIG_MULTIUSER=n kernels have no cap_get_proc()
+		 * Allow them to continue and attempt to access MSRs
+		 */
+		if (errno == ENOSYS)
+			return 0;
+
 		return 1;
+	}
 
 	if (cap_get_flag(caps, CAP_SYS_RAWIO, CAP_EFFECTIVE, &cap_flag_value)) {
 		ret = 1;
-- 
2.39.5