Check whether FS is a real procfs where it matters

For some behaviors there is no substitution for real `/proc`.

One example is `stat()` for `/proc/<pid>/fd` returning the number
of open files for `procfs`. The field has a different meaning on
regular filesystems and we need to check for it in tests.

Work around this by carrying the fact that `FS` is real in the `FS`
struct itself, so that it can be checked where it matters. The realness
is determined once per `FS` creation by doing `statfs()` and checking
for the `PROC_SUPER_MAGIC` match in the returned fs type.

Signed-off-by: Ivan Babrou <[email protected]>

Author: bobrik

fs.go

@@ -14,13 +14,16 @@
 package procfs
 
 import (
+	"syscall"
+
 	"github.com/prometheus/procfs/internal/fs"
 )
 
 // FS represents the pseudo-filesystem sys, which provides an interface to
 // kernel data structures.
 type FS struct {
 	proc fs.FS
+	real bool
 }
 
 // DefaultMountPoint is the common mount point of the proc filesystem.
@@ -39,5 +42,23 @@ func NewFS(mountPoint string) (FS, error) {
 	if err != nil {
 		return FS{}, err
 	}
-	return FS{fs}, nil
+
+	real, err := isRealProc(mountPoint)
+	if err != nil {
+		return FS{}, err
+	}
+
+	return FS{fs, real}, nil
+}
+
+// isRealProc determines whether supplied mountpoint is really a proc filesystem.
+func isRealProc(mountPoint string) (bool, error) {
+	stat := syscall.Statfs_t{}
+	err := syscall.Statfs(mountPoint, &stat)
+	if err != nil {
+		return false, err
+	}
+
+	// 0x9fa0 is PROC_SUPER_MAGIC: https://elixir.bootlin.com/linux/v6.1/source/include/uapi/linux/magic.h#L87
+	return stat.Type == 0x9fa0, nil
 }

proc.go

@@ -21,7 +21,6 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/prometheus/procfs/internal/fs"
 	"github.com/prometheus/procfs/internal/util"
 )
 
@@ -30,7 +29,7 @@ type Proc struct {
 	// The process ID.
 	PID int
 
-	fs fs.FS
+	fs FS
 }
 
 // Procs represents a list of Proc structs.
@@ -92,7 +91,7 @@ func (fs FS) Proc(pid int) (Proc, error) {
 	if _, err := os.Stat(fs.proc.Path(strconv.Itoa(pid))); err != nil {
 		return Proc{}, err
 	}
-	return Proc{PID: pid, fs: fs.proc}, nil
+	return Proc{PID: pid, fs: fs}, nil
 }
 
 // AllProcs returns a list of all currently available processes.
@@ -114,7 +113,7 @@ func (fs FS) AllProcs() (Procs, error) {
 		if err != nil {
 			continue
 		}
-		p = append(p, Proc{PID: int(pid), fs: fs.proc})
+		p = append(p, Proc{PID: int(pid), fs: fs})
 	}
 
 	return p, nil
@@ -238,14 +237,16 @@ func (p Proc) FileDescriptorTargets() ([]string, error) {
 // a process.
 func (p Proc) FileDescriptorsLen() (int, error) {
 	// Use fast path if available (Linux v6.2): https://github.com/torvalds/linux/commit/f1f1f2569901
-	stat, err := os.Stat(p.path("fd"))
-	if err != nil {
-		return 0, err
-	}
+	if p.fs.real {
+		stat, err := os.Stat(p.path("fd"))
+		if err != nil {
+			return 0, err
+		}
 
-	size := stat.Size()
-	if size > 0 {
-		return int(size), nil
+		size := stat.Size()
+		if size > 0 {
+			return int(size), nil
+		}
 	}
 
 	fds, err := p.fileDescriptors()
@@ -296,7 +297,7 @@ func (p Proc) fileDescriptors() ([]string, error) {
 }
 
 func (p Proc) path(pa ...string) string {
-	return p.fs.Path(append([]string{strconv.Itoa(p.PID)}, pa...)...)
+	return p.fs.proc.Path(append([]string{strconv.Itoa(p.PID)}, pa...)...)
 }
 
 // FileDescriptorsInfo retrieves information about all file descriptors of

proc_stat.go

@@ -18,7 +18,6 @@ import (
 	"fmt"
 	"os"
 
-	"github.com/prometheus/procfs/internal/fs"
 	"github.com/prometheus/procfs/internal/util"
 )
 
@@ -112,7 +111,7 @@ type ProcStat struct {
 	// Aggregated block I/O delays, measured in clock ticks (centiseconds).
 	DelayAcctBlkIOTicks uint64
 
-	proc fs.FS
+	proc FS
 }
 
 // NewStat returns the current status information of the process.
@@ -210,8 +209,7 @@ func (s ProcStat) ResidentMemory() int {
 
 // StartTime returns the unix timestamp of the process in seconds.
 func (s ProcStat) StartTime() (float64, error) {
-	fs := FS{proc: s.proc}
-	stat, err := fs.Stat()
+	stat, err := s.proc.Stat()
 	if err != nil {
 		return 0, err
 	}

thread.go

@@ -54,7 +54,8 @@ func (fs FS) AllThreads(pid int) (Procs, error) {
 		if err != nil {
 			continue
 		}
-		t = append(t, Proc{PID: int(tid), fs: fsi.FS(taskPath)})
+
+		t = append(t, Proc{PID: int(tid), fs: FS{fsi.FS(taskPath), fs.real}})
 	}
 
 	return t, nil
@@ -66,13 +67,13 @@ func (fs FS) Thread(pid, tid int) (Proc, error) {
 	if _, err := os.Stat(taskPath); err != nil {
 		return Proc{}, err
 	}
-	return Proc{PID: tid, fs: fsi.FS(taskPath)}, nil
+	return Proc{PID: tid, fs: FS{fsi.FS(taskPath), fs.real}}, nil
 }
 
 // Thread returns a process for a given TID of Proc.
 func (proc Proc) Thread(tid int) (Proc, error) {
-	tfs := fsi.FS(proc.path("task"))
-	if _, err := os.Stat(tfs.Path(strconv.Itoa(tid))); err != nil {
+	tfs := FS{fsi.FS(proc.path("task")), proc.fs.real}
+	if _, err := os.Stat(tfs.proc.Path(strconv.Itoa(tid))); err != nil {
 		return Proc{}, err
 	}
 	return Proc{PID: tid, fs: tfs}, nil

thread_test.go

@@ -37,7 +37,7 @@ func TestAllThreads(t *testing.T) {
 			t.Errorf("want TID %d, have %d", wantTID, haveTID)
 		}
 		wantFS := fixFS.proc.Path(strconv.Itoa(testPID), "task")
-		haveFS := string(threads[i].fs)
+		haveFS := string(threads[i].fs.proc)
 		if wantFS != haveFS {
 			t.Errorf("want fs %q, have %q", wantFS, haveFS)
 		}