[PATCH v4] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[PATCH v4] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[Prashant Singh]

QueryVariableInfo() is an EFI runtime service that, on some firmware,
takes tens of milliseconds and runs with preemption disabled, stalling
the CPU that services it. efivarfs_statfs() calls it (rate-limited since
commit b2326338dc68 ("efivarfs: Rate limit statfs() handler")) to report
the variable-store used/available capacity, so any statfs(2) -- e.g.
every "df" -- can inject that stall into unrelated latency-sensitive
workloads on the same CPU.

Add a negatable "nostatfs" mount option: with nostatfs, statfs(2) skips
QueryVariableInfo() and reports zero used/available; with statfs it
reports the capacity as before. It defaults to nostatfs on
CONFIG_PREEMPT_RT so real-time kernels do not take the stall out of the
box, but statfs can be passed there to force reporting back on -- e.g.
for tools such as fwupd that need the efivars free space to update Secure
Boot key databases.

The option can also be toggled on a live mount via remount, so reporting
can be enabled only for the duration of a firmware update without
unmounting the boot-time efivarfs mount:

  mount -o remount,statfs   /sys/firmware/efi/efivars   # reporting on
  mount -o remount,nostatfs /sys/firmware/efi/efivars   # reporting off

Tested on an Intel Xeon E5-2628L v4, 6.12 kernel, via
"strace -T -e trace=statfs df" on the efivarfs mount.

Cost of the firmware call (CONFIG_PREEMPT_RT not set, so reporting is
enabled by default). Default mount calls QueryVariableInfo() and returns
the real store capacity, ~66-129 ms per call:

  statfs("/sys/firmware/efi/efivars", {f_type=EFIVARFS_MAGIC,
      f_bsize=1, f_blocks=90024, f_bfree=33387, f_bavail=28267, ...})
      = 0 <0.129124>

With -o nostatfs the firmware call is skipped, capacity reported as
zero, ~11 us per call:

  statfs("/sys/firmware/efi/efivars", {f_type=EFIVARFS_MAGIC,
      f_bsize=1, f_blocks=0, f_bfree=0, f_bavail=0, ...}) = 0 <0.000011>

PREEMPT_RT default and live remount toggle (CONFIG_PREEMPT_RT=y). The
boot-time mount defaults to nostatfs (mount shows nostatfs); no firmware
call, capacity zero:

  statfs(... f_blocks=0, f_bfree=0, f_bavail=0, ...) = 0 <0.000018>

Enabling reporting in place with "mount -o remount,statfs" (mount now
shows statfs) takes the ~70 ms firmware call and returns the real
capacity, without unmounting:

  statfs(... f_blocks=90024, f_bfree=30315, f_bavail=25195, ...)
      = 0 <0.070293>

Disabling it again with "mount -o remount,nostatfs" returns to the fast,
zero-capacity path:

  statfs(... f_blocks=0, f_bfree=0, f_bavail=0, ...) = 0 <0.000014>

An unrelated remount that does not respecify the option preserves it: a
"mount -o remount,rw" after "remount,statfs" leaves the mount showing
statfs.

Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Prashant Singh <singhpra@juniper.net>
---
Changes since v3:
- Drop the show_options "statfs" branch; the absence of "nostatfs" now
  indicates reporting is on (Ard Biesheuvel).
- Drop the uid/gid copies on the remount path; efivarfs_reconfigure()
  applies only nostatfs, so they were dead code (Ard Biesheuvel).

Changes since v2:
- Make the flag negatable (fsparam_flag_no): "statfs" forces reporting
  back on, "nostatfs" skips QueryVariableInfo(). Default stays nostatfs
  on PREEMPT_RT. This lets fwupd get the efivars free space it needs for
  Secure Boot key (KEK/DB/DBX) updates on an RT kernel by mounting with
  -o statfs (Luis Claudio R. Goncalves, Steve McIntyre).
- Support toggling the option on a live mount via remount, so reporting
  can be enabled just for a firmware update without unmounting efivarfs
  (Sebastian Andrzej Siewior). Options not respecified on remount are
  preserved.
- Add PREEMPT_RT + remount test data to the commit message.

Changes since v1:
- Replace the sysctl with a mount option named "nostatfs", per review
  (Ard Biesheuvel).

v1: https://lore.kernel.org/all/20260917071600.5587-1-singhpra@juniper.net/
v2: https://lore.kernel.org/all/20260919044124.8268-1-singhpra@juniper.net/
v3: https://lore.kernel.org/all/20260925113145.7396-1-singhpra@juniper.net/

 Documentation/filesystems/efivarfs.rst | 16 +++++++++++
 fs/efivarfs/internal.h                 |  1 +
 fs/efivarfs/super.c                    | 38 ++++++++++++++++++++++----
 3 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/efivarfs.rst b/Documentation/filesystems/efivarfs.rst
index f646c3f0980f..cd81ee84115b 100644
--- a/Documentation/filesystems/efivarfs.rst
+++ b/Documentation/filesystems/efivarfs.rst
@@ -37,6 +37,22 @@ accidentally.
           |4_bytes_of_attributes + efivar_data|
           +-----------------------------------+
 
+Mount options
+=============
+
+statfs / nostatfs
+      Control whether ``statfs(2)`` reports the variable-store used/available
+      capacity. Obtaining it requires the ``QueryVariableInfo()`` EFI runtime
+      service, which on some firmware takes tens of milliseconds and runs with
+      preemption disabled, stalling the calling CPU. With ``nostatfs`` the call
+      is skipped and ``statfs(2)`` reports zero. The default is to report,
+      except on ``CONFIG_PREEMPT_RT`` where it defaults to ``nostatfs``; pass
+      ``statfs`` there to force reporting back on (e.g. for tools such as fwupd
+      that need the free space for firmware updates). The option can also be
+      flipped on a live mount with ``mount -o remount,statfs`` /
+      ``mount -o remount,nostatfs``, so reporting can be enabled only for the
+      duration of a firmware update without unmounting efivarfs.
+
 *See also:*
 
 - Documentation/admin-guide/acpi/ssdt-overlays.rst
diff --git a/fs/efivarfs/internal.h b/fs/efivarfs/internal.h
index f913b6824289..0cb053884b4b 100644
--- a/fs/efivarfs/internal.h
+++ b/fs/efivarfs/internal.h
@@ -11,6 +11,7 @@
 struct efivarfs_mount_opts {
 	kuid_t uid;
 	kgid_t gid;
+	bool nostatfs;		/* skip QueryVariableInfo() in statfs() */
 };
 
 struct efivarfs_fs_info {
diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 8d33f11db2a1..efeada6e314f 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -74,6 +74,9 @@ static int efivarfs_show_options(struct seq_file *m, struct dentry *root)
 	if (!gid_eq(opts->gid, GLOBAL_ROOT_GID))
 		seq_printf(m, ",gid=%u",
 				from_kgid_munged(&init_user_ns, opts->gid));
+	/* Absence of nostatfs means statfs() reports real capacity. */
+	if (opts->nostatfs)
+		seq_puts(m, ",nostatfs");
 	return 0;
 }
 
@@ -82,13 +85,19 @@ static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	const u32 attr = EFI_VARIABLE_NON_VOLATILE |
 			 EFI_VARIABLE_BOOTSERVICE_ACCESS |
 			 EFI_VARIABLE_RUNTIME_ACCESS;
+	struct efivarfs_fs_info *sfi = dentry->d_sb->s_fs_info;
 	u64 storage_space, remaining_space, max_variable_size;
 	u64 id = huge_encode_dev(dentry->d_sb->s_dev);
 	efi_status_t status;
 
-	/* Some UEFI firmware does not implement QueryVariableInfo() */
+	/*
+	 * Some UEFI firmware does not implement QueryVariableInfo(); the
+	 * nostatfs mount option also disables this (preempt-disabled,
+	 * potentially slow) call entirely, reporting zero used/available.
+	 */
 	storage_space = remaining_space = 0;
-	if (efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
+	if (!READ_ONCE(sfi->mount_opts.nostatfs) &&
+	    efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
 		static DEFINE_RATELIMIT_STATE(_rs, 2 * HZ, 5);
 		static u64 storage, remaining;
 		static DEFINE_SPINLOCK(lock);
@@ -323,12 +332,13 @@ static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor,
 }
 
 enum {
-	Opt_uid, Opt_gid,
+	Opt_uid, Opt_gid, Opt_statfs,
 };
 
 static const struct fs_parameter_spec efivarfs_parameters[] = {
 	fsparam_uid("uid", Opt_uid),
 	fsparam_gid("gid", Opt_gid),
+	fsparam_flag_no("statfs", Opt_statfs),
 	{},
 };
 
@@ -350,6 +360,9 @@ static int efivarfs_parse_param(struct fs_context *fc, struct fs_parameter *para
 	case Opt_gid:
 		opts->gid = result.gid;
 		break;
+	case Opt_statfs:
+		opts->nostatfs = result.negated;
+		break;
 	default:
 		return -EINVAL;
 	}
@@ -402,11 +415,17 @@ static int efivarfs_get_tree(struct fs_context *fc)
 
 static int efivarfs_reconfigure(struct fs_context *fc)
 {
+	struct efivarfs_fs_info *sfi = fc->root->d_sb->s_fs_info;
+	struct efivarfs_fs_info *new_sfi = fc->s_fs_info;
+
 	if (!efivar_supports_writes() && !(fc->sb_flags & SB_RDONLY)) {
 		pr_err("Firmware does not support SetVariableRT. Can not remount with rw\n");
 		return -EINVAL;
 	}
 
+	/* statfs() reads nostatfs without s_umount; mark the concurrent update. */
+	WRITE_ONCE(sfi->mount_opts.nostatfs, new_sfi->mount_opts.nostatfs);
+
 	return 0;
 }
 
@@ -524,8 +543,17 @@ static int efivarfs_init_fs_context(struct fs_context *fc)
 	if (!sfi)
 		return -ENOMEM;
 
-	sfi->mount_opts.uid = GLOBAL_ROOT_UID;
-	sfi->mount_opts.gid = GLOBAL_ROOT_GID;
+	if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE) {
+		/* nostatfs is writable via remount; keep it if not respecified. */
+		struct efivarfs_fs_info *old = fc->root->d_sb->s_fs_info;
+
+		sfi->mount_opts.nostatfs = READ_ONCE(old->mount_opts.nostatfs);
+	} else {
+		sfi->mount_opts.uid = GLOBAL_ROOT_UID;
+		sfi->mount_opts.gid = GLOBAL_ROOT_GID;
+		/* QueryVariableInfo() stalls the CPU; default nostatfs on PREEMPT_RT. */
+		sfi->mount_opts.nostatfs = IS_ENABLED(CONFIG_PREEMPT_RT);
+	}
 
 	fc->s_fs_info = sfi;
 	fc->ops = &efivarfs_context_ops;

base-commit: 9b87fdc9af2fbfcdb5c24a64139685ef80f6573f
-- 
2.34.1

Re: [PATCH v4] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] vfs: missing READ_ONCE() for opts->nostatfs in efivarfs_show_options()
--

commit 8539129e21537b2431de520ca79339783ffada2d
Author: Prashant Singh <singhpra@juniper.net>
Subject: efivarfs: add nostatfs mount option to skip QueryVariableInfo()

This commit adds a negatable "nostatfs" mount option to efivarfs. It skips
the EFI QueryVariableInfo() call during statfs(2), which can take tens of
milliseconds and run with preemption disabled. The option can also be toggled
on a live mount via remount.

> diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
> index 8d33f11db2a1b..efeada6e314f4 100644
> --- a/fs/efivarfs/super.c
> +++ b/fs/efivarfs/super.c
> @@ -74,6 +74,9 @@ static int efivarfs_show_options(struct seq_file *m, struct dentry *root)
>  	if (!gid_eq(opts->gid, GLOBAL_ROOT_GID))
>  		seq_printf(m, ",gid=%u",
>  				from_kgid_munged(&init_user_ns, opts->gid));
> +	/* Absence of nostatfs means statfs() reports real capacity. */
> +	if (opts->nostatfs)
       ^^^^

[Severity: Low]
Is a READ_ONCE() needed here?

This plain C read of opts->nostatfs in efivarfs_show_options() can race with
the concurrent WRITE_ONCE() in efivarfs_reconfigure() during a remount. 

A userspace process reading /proc/mounts calls efivarfs_show_options() holding
only namespace_sem for read, while a concurrent mount -o remount calls
efivarfs_reconfigure() holding only sb->s_umount for write. Because no common
lock serializes these two paths, missing the READ_ONCE() here could lead to a
KCSAN data race splat.

> +		seq_puts(m, ",nostatfs");
>  	return 0;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260928203445.72318-1-singhpra@juniper.net?part=1