On Fri, Dec 1, 2023 at 4:42 AM Philippe Mathieu-Daudé <phi...@linaro.org> wrote:
> Hi Shu-Chun,
>
> On 1/12/23 04:21, Shu-Chun Weng wrote:
> > Commit b8002058 strengthened openat()'s /proc detection by calling
> > realpath(3) on the given path, which allows various paths and symlinks
> > that points to the /proc file system to be intercepted correctly.
> >
> > Using realpath(3), though, has a side effect that it reads the symlinks
> > along the way, and thus changes their atime. The results in the
> > following code snippet already get ~now instead of the real atime:
> >
> > int fd = open("/path/to/a/symlink", O_PATH | O_NOFOLLOW);
> > struct stat st;
> > fstat(fd, st);
> > return st.st_atime;
> >
> > This change opens a path that doesn't appear to be part of /proc
> > directly and checks the destination of /proc/self/fd/n to determine if
> > it actually refers to a file in /proc.
> >
> > Neither this nor the existing code works with symlinks or indirect paths
> > (e.g. /tmp/../proc/self/exe) that points to /proc/self/exe because it
> > is itself a symlink, and both realpath(3) and /proc/self/fd/n will
> > resolve into the location of QEMU.
>
> Does this fix any of the following issues?
> https://gitlab.com/qemu-project/qemu/-/issues/829
Not this one -- this is purely in the logic of util/path.c, which we do see
and carry an internal patch. It's quite a behavior change so we never
upstreamed it.
> https://gitlab.com/qemu-project/qemu/-/issues/927
No, either. This patch only touches the path handling, not how files are
opened.
https://gitlab.com/qemu-project/qemu/-/issues/2004
Yes! Though I don't have a toolchain for HPPA or any of the architectures
intercepting /proc/cpuinfo handy, I hacked the condition and confirmed that
on 7.1 and 8.2, test.c as attached in the bug prints out the host cpuinfo
while with this patch, it prints out the content generated by
`open_cpuinfo()`.
>
>
> > Signed-off-by: Shu-Chun Weng <s...@google.com>
>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2004
> ---
> > linux-user/syscall.c | 42 +++++++++++++++++++++++++++++++++---------
> > 1 file changed, 33 insertions(+), 9 deletions(-)
>
>
On Fri, Dec 1, 2023 at 9:09 AM Helge Deller <del...@gmx.de> wrote:
> On 12/1/23 04:21, Shu-Chun Weng wrote:
> > Commit b8002058 strengthened openat()'s /proc detection by calling
> > realpath(3) on the given path, which allows various paths and symlinks
> > that points to the /proc file system to be intercepted correctly.
> >
> > Using realpath(3), though, has a side effect that it reads the symlinks
> > along the way, and thus changes their atime.
>
> Ah, ok. I didn't thought of that side effect when I came up with the patch.
> Does the updated atimes trigger some real case issue ?
>
We have an internal library shimming the underlying filesystem that uses
the `open(O_PATH|O_NOFOLLOW)`+`fstat()` pattern for all file stats.
Checking symlink atime is in one of the unittests, though I don't know if
production ever uses it.
>
> Helge
>
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