[PATCH] ext4: Give 32bit personalities 32bit hashes

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

It was brought to my attention that this bug from 2018 was
still unresolved: 32 bit emulators like QEMU were given
64 bit hashes when running 32 bit emulation on 64 bit systems.

The personality(2) system call supports to let processes
indicate that they are 32 bit Linux to the kernel. This
was suggested by Teo in the original thread, so I just wired
it up and it solves the problem.

Programs that need the 32 bit hash only need to issue the
personality(PER_LINUX32) call and things start working.

I made a test program like this:

  #include <dirent.h>
  #include <errno.h>
  #include <stdio.h>
  #include <string.h>
  #include <sys/types.h>
  #include <sys/personality.h>

  int main(int argc, char** argv) {
    DIR* dir;
    personality(PER_LINUX32);
    dir = opendir("/boot");
    printf("dir=%p\n", dir);
    printf("readdir(dir)=%p\n", readdir(dir));
    printf("errno=%d: %s\n", errno, strerror(errno));
    return 0;
  }

This was compiled with an ARM32 toolchain from Bootlin using
glibc 2.28 and thus suffering from the bug.

Before the patch:

  $ ./readdir-bug
  dir=0x86000
  readdir(dir)=(nil)
  errno=75: Value too large for defined data type

After the patch:

  $ ./readdir-bug
  dir=0x86000
  readdir(dir)=0x86020
  errno=0: Success

Problem solved.

Cc: Florian Weimer <fw@xxxxxxxxxxxxx>
Cc: Peter Maydell <peter.maydell@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Theodore Ts'o <tytso@xxxxxxx>
Link: https://bugs.launchpad.net/qemu/+bug/1805913
Link: https://lore.kernel.org/lkml/87bm56vqg4.fsf@xxxxxxxxxxxxxxxxx/
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=205957
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
 fs/ext4/dir.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 9aa1f75409b0..3faf9edf3e92 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -27,6 +27,7 @@
 #include <linux/slab.h>
 #include <linux/iversion.h>
 #include <linux/unicode.h>
+#include <linux/personality.h>
 #include "ext4.h"
 #include "xattr.h"
 
@@ -618,6 +619,14 @@ static int ext4_dx_readdir(struct file *file, struct dir_context *ctx)
 
 static int ext4_dir_open(struct inode * inode, struct file * filp)
 {
+	/*
+	 * If we are currently running e.g. a 32 bit emulator on
+	 * a 64 bit machine, the emulator will indicate that it needs
+	 * a 32 bit personality and thus 32 bit hashes from the file
+	 * system.
+	 */
+	if (personality(current->personality) == PER_LINUX32)
+		filp->f_mode |= FMODE_32BITHASH;
 	if (IS_ENCRYPTED(inode))
 		return fscrypt_get_encryption_info(inode) ? -EACCES : 0;
 	return 0;
-- 
2.24.1