Don't enforce NAME_MAX in fs.Dirent.walk().

Maximum filename length is filesystem-dependent, and obtained via
statfs::f_namelen. This limit is usually 255 bytes (NAME_MAX), but not
always. For example, VFAT supports filenames of up to 255... UCS-2
characters, which Linux conservatively takes to mean UTF-8-encoded
bytes: fs/fat/inode.c:fat_statfs(), FAT_LFN_LEN * NLS_MAX_CHARSET_SIZE.
As a result, Linux's VFS does not enforce NAME_MAX:

$ rg --maxdepth=1 '\WNAME_MAX\W' fs/ include/linux/
fs/libfs.c
38:     buf->f_namelen = NAME_MAX;
64:     if (dentry->d_name.len > NAME_MAX)

include/linux/relay.h
74:     char base_filename[NAME_MAX];   /* saved base filename */

include/linux/fscrypt.h
149: * filenames up to NAME_MAX bytes, since base64 encoding expands the length.

include/linux/exportfs.h
176: *    understanding that it is already pointing to a a %NAME_MAX+1 sized

Remove this check from core VFS, and add it to ramfs (and by extension
tmpfs), where it is actually applicable:
mm/shmem.c:shmem_dir_inode_operations.lookup == simple_lookup *does*
enforce NAME_MAX.

PiperOrigin-RevId: 245324748
Change-Id: I17567c4324bfd60e31746a5270096e75db963fac

2 files changed, 1 insertions, 5 deletions

diff --git a/test/syscalls/linux/chdir.cc b/test/syscalls/linux/chdir.cc
index 4905ffb23..a4b54f0ee 100644
--- a/test/syscalls/linux/chdir.cc
+++ b/test/syscalls/linux/chdir.cc
@@ -54,11 +54,6 @@ TEST(ChdirTest, NotDir) {
   EXPECT_THAT(chdir(temp_file.path().c_str()), SyscallFailsWithErrno(ENOTDIR));
 }
 
-TEST(ChdirTest, NameTooLong) {
-  std::string name(NAME_MAX + 1, 'a');
-  ASSERT_THAT(chdir(name.c_str()), SyscallFailsWithErrno(ENAMETOOLONG));
-}
-
 TEST(ChdirTest, NotExist) {
   EXPECT_THAT(chdir("/foo/bar"), SyscallFailsWithErrno(ENOENT));
 }
diff --git a/test/syscalls/linux/statfs.cc b/test/syscalls/linux/statfs.cc
index 1fc9758c9..e1e7fc707 100644
--- a/test/syscalls/linux/statfs.cc
+++ b/test/syscalls/linux/statfs.cc
@@ -49,6 +49,7 @@ TEST(StatfsTest, NameLen) {
   struct statfs st;
   EXPECT_THAT(statfs("/dev/shm", &st), SyscallSucceeds());
 
+  // This assumes that /dev/shm is tmpfs.
   EXPECT_EQ(st.f_namelen, NAME_MAX);
 }