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

["Rainer Müller" <raimue@codingfarm.de>]

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
 
         if ((r = fake_open->fill(cpu_env, fd))) {
             int e = errno;
-- 
2.25.1