[PATCH AUTOSEL 6.16-6.12] tools/power turbostat: Handle non-root legacy-uncore sysfs permissions

[Sasha Levin <sashal@kernel.org>]

From: Len Brown <len.brown@intel.com>

[ Upstream commit e60a13bcef206795d3ddf82f130fe8f570176d06 ]

/sys/devices/system/cpu/intel_uncore_frequency/package_X_die_Y/
may be readable by all, but
/sys/devices/system/cpu/intel_uncore_frequency/package_X_die_Y/current_freq_khz
may be readable only by root.

Non-root turbostat users see complaints in this scenario.

Fail probe of the interface if we can't read current_freq_khz.

Reported-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Original-patch-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

Now I have a clear understanding of the issue and fix. Let me analyze
the code change:

## Backport Status: **YES**

## Extensive Analysis:

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

### 1. **Bug Fix for Real User Issue**
The commit fixes a specific bug where non-root users of turbostat
encounter errors when the sysfs permissions are inconsistent. As stated
in the commit message:
- The directory
  `/sys/devices/system/cpu/intel_uncore_frequency/package_X_die_Y/` may
  be readable by all users
- But the file `current_freq_khz` within that directory may only be
  readable by root
- This causes turbostat to fail with complaints for non-root users

### 2. **Simple and Contained Fix**
The code change is minimal and focused:
```c
// Before: Only checked if the directory was readable
if (access(path_base, R_OK))
    continue;

// After: Check if the actual file we need (current_freq_khz) is
readable
sprintf(path, "%s/current_freq_khz", path_base);
if (access(path, R_OK))
    continue;
```

The fix simply moves the permission check from the directory to the
actual file that will be read later (line 6763-6765 in the original code
reads `current_freq_khz`).

### 3. **Follows Established Pattern**
Looking at the `probe_intel_uncore_frequency_cluster()` function (line
6776), it already checks the specific file permission:
```c
if (access("/sys/devices/system/cpu/intel_uncore_frequency/uncore00/curr
ent_freq_khz", R_OK))
    return;
```
This fix makes the legacy function consistent with the cluster
function's approach.

### 4. **No Risk of Regression**
- The change only affects the probing logic - if the file cannot be
  read, the function simply skips that uncore frequency entry
- This is better than the current behavior where it would try to read
  the file anyway and fail with errors
- The fix maintains backward compatibility - systems where the file is
  readable continue to work as before

### 5. **Userspace Tool Fix**
This is a fix for a userspace tool (turbostat) in the tools/ directory,
not kernel code, making it even safer to backport as it cannot cause
kernel instability.

### 6. **Clear Problem and Solution**
- The problem is well-defined and reported by a real user (Artem
  Bityutskiy)
- The solution is straightforward and logical
- The fix prevents error messages that would confuse users running
  turbostat without root privileges

### 7. **Meets Stable Criteria**
According to stable kernel rules, this fix:
- Fixes a real bug that affects users ✓
- Is small and contained (2 line change) ✓
- Has minimal risk of introducing new issues ✓
- Doesn't introduce new features ✓
- Doesn't make architectural changes ✓

The commit improves the robustness of turbostat when dealing with
permission inconsistencies in sysfs, which is a reasonable scenario that
can occur in production systems with specific security configurations.

 tools/power/x86/turbostat/turbostat.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/power/x86/turbostat/turbostat.c b/tools/power/x86/turbostat/turbostat.c
index 5230e072e414..bf011c2847f2 100644
--- a/tools/power/x86/turbostat/turbostat.c
+++ b/tools/power/x86/turbostat/turbostat.c
@@ -6740,7 +6740,8 @@ static void probe_intel_uncore_frequency_legacy(void)
 			sprintf(path_base, "/sys/devices/system/cpu/intel_uncore_frequency/package_%02d_die_%02d", i,
 				j);
 
-			if (access(path_base, R_OK))
+			sprintf(path, "%s/current_freq_khz", path_base);
+			if (access(path, R_OK))
 				continue;
 
 			BIC_PRESENT(BIC_UNCORE_MHZ);
-- 
2.39.5