Support setting STATX_SIZE for kernfs.InodeAttrs.

Make setting STATX_SIZE a no-op, if it is valid for the given permissions and
file type.

Also update proc tests, which were overfitted before.

Fixes #3842.
Updates #1193.

PiperOrigin-RevId: 331861087

2 files changed, 17 insertions, 25 deletions

diff --git a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
index c0b863ba4..74408e322 100644
--- a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
@@ -259,9 +259,19 @@ func (a *InodeAttrs) SetStat(ctx context.Context, fs *vfs.Filesystem, creds *aut
 	if opts.Stat.Mask == 0 {
 		return nil
 	}
-	if opts.Stat.Mask&^(linux.STATX_MODE|linux.STATX_UID|linux.STATX_GID) != 0 {
+
+	// Note that not all fields are modifiable. For example, the file type and
+	// inode numbers are immutable after node creation. Setting the size is often
+	// allowed by kernfs files but does not do anything. If some other behavior is
+	// needed, the embedder should consider extending SetStat.
+	//
+	// TODO(gvisor.dev/issue/1193): Implement other stat fields like timestamps.
+	if opts.Stat.Mask&^(linux.STATX_MODE|linux.STATX_UID|linux.STATX_GID|linux.STATX_SIZE) != 0 {
 		return syserror.EPERM
 	}
+	if opts.Stat.Mask&linux.STATX_SIZE != 0 && a.Mode().IsDir() {
+		return syserror.EISDIR
+	}
 	if err := vfs.CheckSetStat(ctx, creds, &opts, a.Mode(), auth.KUID(atomic.LoadUint32(&a.uid)), auth.KGID(atomic.LoadUint32(&a.gid))); err != nil {
 		return err
 	}
@@ -284,13 +294,6 @@ func (a *InodeAttrs) SetStat(ctx context.Context, fs *vfs.Filesystem, creds *aut
 		atomic.StoreUint32(&a.gid, stat.GID)
 	}
 
-	// Note that not all fields are modifiable. For example, the file type and
-	// inode numbers are immutable after node creation.
-
-	// TODO(gvisor.dev/issue/1193): Implement other stat fields like timestamps.
-	// Also, STATX_SIZE will need some special handling, because read-only static
-	// files should return EIO for truncate operations.
-
 	return nil
 }
 
diff --git a/test/syscalls/linux/proc.cc b/test/syscalls/linux/proc.cc
index f3f80a12a..1ba7bf4f5 100644
--- a/test/syscalls/linux/proc.cc
+++ b/test/syscalls/linux/proc.cc
@@ -797,17 +797,12 @@ TEST(ProcCpuinfo, DeniesWriteNonRoot) {
     constexpr int kNobody = 65534;
     EXPECT_THAT(syscall(SYS_setuid, kNobody), SyscallSucceeds());
     EXPECT_THAT(open("/proc/cpuinfo", O_WRONLY), SyscallFailsWithErrno(EACCES));
-    // TODO(gvisor.dev/issue/1193): Properly support setting size attributes in
-    // kernfs.
-    if (!IsRunningOnGvisor() || IsRunningWithVFS1()) {
-      EXPECT_THAT(truncate("/proc/cpuinfo", 123),
-                  SyscallFailsWithErrno(EACCES));
-    }
+    EXPECT_THAT(truncate("/proc/cpuinfo", 123), SyscallFailsWithErrno(EACCES));
   });
 }
 
 // With root privileges, it is possible to open /proc/cpuinfo with write mode,
-// but all write operations will return EIO.
+// but all write operations should fail.
 TEST(ProcCpuinfo, DeniesWriteRoot) {
   // VFS1 does not behave differently for root/non-root.
   SKIP_IF(IsRunningWithVFS1());
@@ -816,16 +811,10 @@ TEST(ProcCpuinfo, DeniesWriteRoot) {
   int fd;
   EXPECT_THAT(fd = open("/proc/cpuinfo", O_WRONLY), SyscallSucceeds());
   if (fd > 0) {
-    EXPECT_THAT(write(fd, "x", 1), SyscallFailsWithErrno(EIO));
-    EXPECT_THAT(pwrite(fd, "x", 1, 123), SyscallFailsWithErrno(EIO));
-  }
-  // TODO(gvisor.dev/issue/1193): Properly support setting size attributes in
-  // kernfs.
-  if (!IsRunningOnGvisor() || IsRunningWithVFS1()) {
-    if (fd > 0) {
-      EXPECT_THAT(ftruncate(fd, 123), SyscallFailsWithErrno(EIO));
-    }
-    EXPECT_THAT(truncate("/proc/cpuinfo", 123), SyscallFailsWithErrno(EIO));
+    // Truncate is not tested--it may succeed on some kernels without doing
+    // anything.
+    EXPECT_THAT(write(fd, "x", 1), SyscallFails());
+    EXPECT_THAT(pwrite(fd, "x", 1, 123), SyscallFails());
   }
 }