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

From: Dmitry V. Levin
Date: Thu Dec 27 2018 - 21:24:01 EST

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


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