DriveInfo.Linux: use procfs mountinfo for formats and mount point paths. #116102

tmds · 2025-05-29T16:49:21Z

For the format, mountinfo includes a string 'filesystem type' field which is more granular than the statfs f_type numeric field. For example: the ext2, ext3 and ext4 are distinct mountinfo values which have the same f_type value.

For the mount paths, mountinfo provides the mount point paths relative to the process's root directory. When the process runs with changed root dir (chroot), the paths are adjusted for it.

This is a breaking change. On Linux, the DriveInfo.DriveFormat previously were names from the UnixFileSystemTypes enum members. With this change, they will be Linux kernel filesystem type strings.

Fixes #95099.

@dotnet/area-system-io @stephentoub ptal.

For the format, mountinfo includes a string 'filesystem type' field which is more granular than the statfs f_type numeric field. For example: the ext2, ext3 and ext4 are distinct mountinfo values which have the same same f_type value. For the mount paths, mountinfo provides the mount point paths relative to the process's root directory. When the process runs with changed root dir (chroot), the paths are adjusted for it.

tmds · 2025-05-29T16:54:46Z

src/libraries/Common/src/Interop/Linux/cgroups/Interop.cgroups.cs

-            long numericFormat;
-            int result = Interop.Sys.GetFormatInfoForMountPoint(SysFsCgroupFileSystemPath, formatBuffer, MountPointFormatBufferSizeInBytes, &numericFormat);
-            if (result == 0)
+            Span<char> buffer = stackalloc char[16];


This file was also parsing mountinfo, so I've updated it to share the parsing logic with DriveInfo.

src/native/libs/System.Native/pal_mount.c

src/libraries/Common/src/Interop/Linux/procfs/Interop.ProcMountInfo.cs

src/libraries/Common/src/Interop/Unix/System.Native/Interop.MountPoints.FormatInfo.cs

tmds · 2025-06-02T06:55:29Z

src/libraries/Common/src/Interop/Unix/System.Native/Interop.MountPoints.FormatInfo.cs

-                    Marshal.PtrToStringUTF8((IntPtr)formatBuffer)!;
-                type = GetDriveType(format);
+                // Canonicalize and resolve symbolic links.
+                string? path = Sys.RealPath(name);


This function is now going to make several syscalls where there used to be one.

If this turns out to be an performance issue, we can reduce the nr of calls:

If we use statx then we can get the STATX_MNT_ID which corresponds to the first field in mountinfo. This replaces multiple syscalls made by realpath into a single syscall.

For Linux 6.8+, we can call statx with STATX_MNT_ID_UNIQUE and then use that with the statmount syscall with STATMOUNT_FS_TYPE which avoids the syscalls for opening and reading mountinfo to get the filesystem type name.

I don't intend to include these changes as part of this PR (unless someone finds it important).

tmds · 2025-06-02T06:55:56Z

@dotnet/area-system-io this is up for review.

am11 · 2025-06-02T07:44:54Z

@tmds, was the refactoring on native side essential for this to work? I am asking because we implemented the fallbacks when I was on a "many different OS / filesystems configuration testing" trips (Android emulator on macOS vs. Windows, illumos various filesystems ZFS-based fs vs. the others, freebsd UFS vs. ZFS-based fs etc. etc.) and it took some time to figure out the magic values (this is the only place on the entire GitHub where we find such an exhaustive list of filesystem magic vals).

I am pretty sure you have kept them logically equivalent, but on an off chance refactoring might break some prior contract, I suggest we keep this implementation to Linux only on C# side and refactor stuff in a separate PR (or leave it be to avoid going through laborious testing).

tmds · 2025-06-02T08:37:12Z

@tmds, was the refactoring on native side essential for this to work? I am asking because we implemented the fallbacks when I was on a "many different OS / filesystems configuration testing" trips (Android emulator on macOS vs. Windows, illumos various filesystems ZFS-based fs vs. the others, freebsd UFS vs. ZFS-based fs etc. etc.) and it took some time to figure out the magic values (this is the only place on the entire GitHub where we find such an exhaustive list of filesystem magic vals).

This is about #116102 (comment).

The main problem with UnixFileSystemTypes is that it should be split per OS because these magic values are not cross-platform.

I removed it because I assumed it was only used for Linux mapping for which the PR is now using mountinfo.

I'll add back the f_type + UnixFileSystemTypes mapping.

…and remove duplicate value members.

tmds · 2025-06-04T05:45:38Z

This is up for review.

tmds · 2025-06-04T06:32:43Z

src/libraries/Common/src/Interop/Linux/procfs/Interop.ProcMountInfo.TryParseMountInfoLine.cs

+            }
+            line = line.Slice(endOfOptionalFields + Separator.Length);
+            fields = line.Split(' ');
+
+            // (9) filesystem type
+            if (!fields.MoveNext())


Each block handles the field that is in the comment at the first line.
// (8) separator updates fields to be past the separator.

tmds · 2025-06-04T06:32:43Z

src/libraries/Common/src/Interop/Linux/procfs/Interop.ProcMountInfo.TryParseMountInfoLine.cs

+            }
+            line = line.Slice(endOfOptionalFields + Separator.Length);
+            fields = line.Split(' ');
+
+            // (9) filesystem type
+            if (!fields.MoveNext())


Each block handles the field that is in the comment at the first line.
// (8) separator updates fields to be past the separator.

tmds · 2025-06-09T04:34:51Z

@dotnet/area-system-io ptal.

tmds · 2025-06-16T06:30:11Z

@jeffhandley @adamsitnik can you take a look? It would be nice if this gets some usage in preview7.

jozkee

Otherwise, LGTM. Thanks, @tmds.

jozkee · 2025-06-04T16:50:07Z

src/libraries/Common/src/Interop/Unix/System.Native/Interop.MountPoints.FormatInfo.cs

+                string? path = Sys.RealPath(name);
+                if (path is null)
+                {
+                    format = "";


string.Empty is used on ln 68.

Suggested change

format = "";

format = string.Empty;

jozkee · 2025-06-04T16:56:28Z

src/libraries/Common/src/Interop/Linux/procfs/Interop.ProcMountInfo.cs

+            if (File.Exists(ProcMountInfoFilePath))
+            {
+                try
+                {


Reduce nesting.

Suggested change

if (File.Exists(ProcMountInfoFilePath))

{

try

{

if (!File.Exists(ProcMountInfoFilePath))

return Error.ENOTSUP

try

{

jozkee · 2025-06-16T17:22:44Z

src/libraries/Common/src/Interop/Unix/System.Native/Interop.UnixFileSystemTypes.cs

-        /// where this enum must be a subset of the GetDriveType list, with the enum
-        /// values here exactly matching a string there.
+        /// where this enum must be a subset of the GetDriveType list, with the enum values here exactly matching a string there.
+        /// If possible, use names that match the Linux file system type strings.


?

Suggested change

/// If possible, use names that match the Linux file system type strings.

/// If possible, use names that match the Linux file system type strings in /proc/filesystems.

jozkee · 2025-06-16T17:25:03Z

src/libraries/Common/src/Interop/Unix/System.Native/Interop.MountPoints.cs

+            if (OperatingSystem.IsLinux())
+            {
+                if (File.Exists(Interop.procfs.ProcMountInfoFilePath))


Suggested change

if (OperatingSystem.IsLinux())

{

if (File.Exists(Interop.procfs.ProcMountInfoFilePath))

if (OperatingSystem.IsLinux() && File.Exists(Interop.procfs.ProcMountInfoFilePath))

jozkee · 2025-06-16T17:36:19Z

src/libraries/Common/src/Interop/Unix/System.Native/Interop.UnixFileSystemTypes.cs

-            selinux = 0xF97CFF8C,
-            sffs = 0x786F4256, // same as vboxfs
-            sharefs = 0x01021994, // same as tmpfs
+            selinuxfs = 0xF97CFF8C,


All these changes (add/del fs suffix) seem to be worth including in the breaking change doc.

jozkee · 2025-06-16T23:01:17Z

/ba-g

build failure for browser-wasm windows Release LibraryTests FAILED: mono/metadata/CMakeFiles/metadata_objects.dir/mono-hash.c.o, unrealated, these changes are for Unix.
System.Runtime.Tests.WorkItemExecution: DeadLetter
Wasm.Build.NativeRebuild.Tests.ReferenceNewAssemblyRebuildTest.ReferenceNewAssembly - Wasm.Build.Tests timeout #116697

Merging to include in preview6.

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tmds commented May 29, 2025

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tmds mentioned this pull request May 29, 2025

Fix wrong drive format on Unix #107365

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tmds commented May 29, 2025

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src/native/libs/System.Native/pal_mount.c Outdated Show resolved Hide resolved

tmds added area-System.IO and removed needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners labels May 29, 2025

jkotas reviewed May 29, 2025

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src/libraries/Common/src/Interop/Linux/procfs/Interop.ProcMountInfo.cs Outdated Show resolved Hide resolved

Remove stackalloced buffer.

9c7b790

build-analysis bot mentioned this pull request May 29, 2025

SIGKILL System.Runtime.Serialization.Formatters.Tests #114287

Open

Resolve symbolic links.

7f679e0

stephentoub added the breaking-change Issue or PR that represents a breaking API or functional change over a prerelease. label May 29, 2025

dotnet-policy-service bot added the needs-breaking-change-doc-created Breaking changes need an issue opened with https://github.com/dotnet/docs/issues/new?template=dotnet label May 29, 2025

Fix Common.Tests build on non-Linux.

83995f0

kasperk81 reviewed May 30, 2025

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src/libraries/Common/src/Interop/Unix/System.Native/Interop.MountPoints.FormatInfo.cs Show resolved Hide resolved

build-analysis bot mentioned this pull request May 30, 2025

Test failure: System.Reflection.Metadata.ApplyUpdateTest.TestGenericAddStaticField #115318

Open

tmds added 5 commits May 30, 2025 12:22

Add test that gets the DriveFormat for /tmp.

3ae8d7a

Use test to debug CI failure.

72dff48

Fix check for root mount path.

d86e0af

pal_mount: use buffer length based on API used.

a8cc5cb

Fix not calling native GetFileSystemTypeNameForMountPoint on non-Linux.

7c7ebb2

tmds commented Jun 2, 2025

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Add back using 'f_type' when 'f_fstypename' is not available.

a18f097

tmds added 2 commits June 2, 2025 14:14

UnixFileSystemTypes: prefer Linux file system type strings as names, …

efd1dfd

…and remove duplicate value members.

Fix wasi build breakage.

4f7e3f2

am11 approved these changes Jun 4, 2025

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tmds commented Jun 4, 2025

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jozkee approved these changes Jun 16, 2025

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jozkee closed this Jun 16, 2025

jozkee reopened this Jun 16, 2025

Merge branch 'main' into drive_info

ab7338c

jozkee mentioned this pull request Jun 16, 2025

[browser] NoFingerprint, LazyLoadingTests.LoadLazyAssemblyBeforeItIsNeeded, timeout #113836

Open

jozkee merged commit 9b493d2 into dotnet:main Jun 16, 2025
154 of 159 checks passed

tmds mentioned this pull request Jun 17, 2025

[Breaking change]: DriveInfo.DriveFormat Linux values. dotnet/docs#46831

Open

3 tasks

AndyAyersMS mentioned this pull request Jun 19, 2025

[Perf] Linux/arm64: 1 Regression on 6/17/2025 12:35:31 AM +00:00 #116826

Open

-            if (File.Exists(ProcMountInfoFilePath))
-            {
-                try
-                {
+            if (!File.Exists(ProcMountInfoFilePath))
+                return Error.ENOTSUP
+            try
+            {

	/// If possible, use names that match the Linux file system type strings.
	/// If possible, use names that match the Linux file system type strings in /proc/filesystems.

DriveInfo.Linux: use procfs mountinfo for formats and mount point paths. #116102

DriveInfo.Linux: use procfs mountinfo for formats and mount point paths. #116102

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