Re: [PATCH 2/2] linux-user: Fix openat() emulation to not modify atime

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

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
https://gitlab.com/qemu-project/qemu/-/issues/927
https://gitlab.com/qemu-project/qemu/-/issues/2004

> Signed-off-by: Shu-Chun Weng <scw@google.com>
> ---
>   linux-user/syscall.c | 42 +++++++++++++++++++++++++++++++++---------
>   1 file changed, 33 insertions(+), 9 deletions(-)