Re: [Qemu-devel] [PATCH] linux-user: Pass through nanosecond timestamp components for stat syscalls

[Laurent Vivier]

On 22/05/2019 11:45, Chen-Yu Tsai wrote:
> On Wed, May 22, 2019 at 5:08 PM Laurent Vivier <address@hidden> wrote:
> > On 14/05/2019 16:53, Chen-Yu Tsai wrote:
> > > From: Chen-Yu Tsai <address@hidden>
> > >
> > > Since Linux 2.6 the stat syscalls have mostly supported nanosecond
> > > components for each of the file-related timestamps.
> > >
> > > QEMU user mode emulation currently does not pass through the nanosecond
> > > portion of the timestamp, even when the host system fills in the value.
> > > This results in a mismatch when run on subsecond resolution filesystems
> > > such as ext4 or XFS.
> > >
> > > An example of this leading to inconsistency is cross-debootstraping a
> > > full desktop root filesystem of Debian Buster. Recent versions of
> > > fontconfig store the full timestamp (instead of just the second portion)
> > > of the directory in its per-directory cache file, and checks this against
> > > the directory to see if the cache is up-to-date. With QEMU user mode
> > > emulation, the timestamp stored is incorrect, and upon booting the rootfs
> > > natively, fontconfig discovers the mismatch, and proceeds to rebuild the
> > > cache on the comparatively slow machine (low-power ARM vs x86). This
> > > stalls the first attempt to open whatever application that incorporates
> > > fontconfig.
> > >
> > > This patch renames the "unused" padding trailing each timestamp element
> > > to its nanosecond counterpart name if such an element exists in the
> > > kernel sources for the given platform. Not all do. Then have the syscall
> > > wrapper fill in the nanosecond portion if the host supports it, as
> > > specified by the _POSIX_C_SOURCE and _XOPEN_SOURCE feature macros.
> > >
> > > Recent versions of glibc only use stat64 and newfstatat syscalls on
> > > 32-bit and 64-bit platforms respectively. The changes in this patch
> > > were tested by directly calling the stat, stat64 and newfstatat syscalls
> > > directly, in addition to the glibc wrapper, on arm and aarch64 little
> > > endian targets.
> > >
> > > Signed-off-by: Chen-Yu Tsai <address@hidden>
> > >
> > > ---
> > >
> > > This issue was found while integrating some software that uses newer
> > > versions of fontconfig into Raspbian images. We found that the first
> > > launch of said software always stalls with fontconfig regenerating its
> > > font cache files. Upon closer examination I found the timestamps were
> > > not matching. The rest is explained above. Currently we're just working
> > > around the problem by patching the correct timestamps into the cache
> > > files after the fact.
> > >
> > > Please consider this a drive-by scratch-my-own-itch contribution, but I
> > > will stick around to deal with any comments raised during review. I'm
> > > not on the mailing lists either, so please keep me in CC.
> > >
> > > checkpatch returns "ERROR: code indent should never use tabs" for
> > > linux-user/syscall_defs.h, however as far as I can tell the whole file
> > > is indented with tabs. I'm not sure what to make of this.
> >
> > Yes, the file is entirely indented with tabs, so you can let this as-is.
> > Anyway, I plan to split the file in several ones so we will be able to
> > swap the tabs with spaces.
> >
> > Reviewed-by: Laurent Vivier <address@hidden>
>
> Thanks. Unfortunately this patch has some issues. It fails to build for
> targets that don't have the *_nsec fields, such as Alpha or M68K.

ok, anyway I always build all linux-user target before my PR, I would 
have seend the problem :)

> I'll spin a v2 with a new macro TARGET_STAT_HAS_NSEC defined for targets
> that have the fields, added before each struct stat definition. The hunk
> below will gain a check against said macro. This is pretty much how the
> kernel deals with the difference as well, as I just found out.
>
> > > @@ -8866,6 +8876,14 @@ static abi_long do_syscall1(void *cpu_env, int num, 
> > > abi_long arg1,
> > >                    __put_user(st.st_atime, &target_st->target_st_atime);
> > >                    __put_user(st.st_mtime, &target_st->target_st_mtime);
> > >                    __put_user(st.st_ctime, &target_st->target_st_ctime);
> > > +#if _POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700
> > > +                __put_user(st.st_atim.tv_nsec,
> > > +                           &target_st->target_st_atime_nsec);
> > > +                __put_user(st.st_mtim.tv_nsec,
> > > +                           &target_st->target_st_mtime_nsec);
> > > +                __put_user(st.st_ctim.tv_nsec,
> > > +                           &target_st->target_st_ctime_nsec);
> > > +#endif
> > >                    unlock_user_struct(target_st, arg2, 1);
> > >                }
> > >            }
>
> If that sounds good to you I'll keep your reviewed-by for v2.

You can.

Thanks,
Laurent