Re: d_off field in struct dirent and 32-on-64 emulation

* Dmitry V. Levin:

On Thu, Dec 27, 2018 at 06:18:19PM +0100, Florian Weimer wrote:

quoted

We have a bit of an interesting problem with respect to the d_off
field in struct dirent.

When running a 64-bit kernel on certain file systems, notably ext4,
this field uses the full 63 bits even for small directories (strace -v
output, wrapped here for readability):

getdents(3, [
  {d_ino=1494304, d_off=3901177228673045825, d_reclen=40,
d_name="authorized_keys", d_type=DT_REG},
  {d_ino=1494277, d_off=7491915799041650922, d_reclen=24,
d_name=".", d_type=DT_DIR},
  {d_ino=1314655, d_off=9223372036854775807, d_reclen=24,
d_name="..", d_type=DT_DIR}
], 32768) = 88

When running in 32-bit compat mode, this value is somehow truncated to
31 bits, for both the getdents and the getdents64 (!) system call (at
least on i386).

Why getdents64 system call is affected by this truncation,
isn't it a kernel bug that has to be fixed in the kernel instead?

It's required because POSIX specifies that telldir and seekdir use
long int (and not off_t) as the seek offset.  If the kernel does not
truncate while keeping a useful value, these functions would turn
unusable.