Re: [PATCH] linux-user: Use memfd for open syscall emulation

[Richard Henderson <richard.henderson@linaro.org>]

On 7/25/22 09:28, Rainer Müller wrote:
> For certain paths in /proc, the open syscall is intercepted and the
> returned file descriptor points to a temporary file with emulated
> contents.
> 
> If TMPDIR is not accessible or writable for the current user (for
> example in a read-only mounted chroot or container) tools such as ps
> from procps may fail unexpectedly. Trying to read one of these paths
> such as /proc/self/stat would return an error such as ENOENT or EROFS.
> 
> To relax the requirement on a writable TMPDIR, use memfd_create()
> instead to create an anonymous file and return its file descriptor.
> 
> Signed-off-by: Rainer Müller <raimue@codingfarm.de>
> ---
>   linux-user/syscall.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 991b85e6b4..3e4af930ad 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -8265,9 +8265,11 @@ static int do_openat(CPUArchState *cpu_env, int dirfd, const char *pathname, int
>       }
>   
>       if (fake_open->filename) {
> +        int fd, r;
> +
> +#ifndef CONFIG_MEMFD
>           const char *tmpdir;
>           char filename[PATH_MAX];
> -        int fd, r;
>   
>           /* create temporary file to map stat to */
>           tmpdir = getenv("TMPDIR");
> @@ -8279,6 +8281,12 @@ static int do_openat(CPUArchState *cpu_env, int dirfd, const char *pathname, int
>               return fd;
>           }
>           unlink(filename);
> +#else
> +        fd = memfd_create("qemu-open", 0);
> +        if (fd < 0) {
> +            return fd;
> +        }
> +#endif

Even without CONFIG_MEMFD, we will have the memfd_create function available in util/.
I think you should drop the ifdefs like so:

#include "qemu/memfd.h"

     fd = memfd_create(...);
     if (fd < 0) {
         if (errno != ENOSYS) {
             return fd;
         }
         // tmpdir fallback
     }


r~