[PATCH v2] 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>
---
v2: no more #ifdefs, use stub from util/memfd.c with ENOSYS fallback,
    tested with 'strace -e fault=memfd_create'
---
 linux-user/syscall.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 991b85e6b4..7b55726f25 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -8269,16 +8269,22 @@ static int do_openat(CPUArchState *cpu_env, int dirfd, const char *pathname, int
         char filename[PATH_MAX];
         int fd, r;
 
-        /* create temporary file to map stat to */
-        tmpdir = getenv("TMPDIR");
-        if (!tmpdir)
-            tmpdir = "/tmp";
-        snprintf(filename, sizeof(filename), "%s/qemu-open.XXXXXX", tmpdir);
-        fd = mkstemp(filename);
+        fd = memfd_create("qemu-open", 0);
         if (fd < 0) {
-            return fd;
+            if (errno != ENOSYS) {
+                return fd;
+            }
+            /* create temporary file to map stat to */
+            tmpdir = getenv("TMPDIR");
+            if (!tmpdir)
+                tmpdir = "/tmp";
+            snprintf(filename, sizeof(filename), "%s/qemu-open.XXXXXX", tmpdir);
+            fd = mkstemp(filename);
+            if (fd < 0) {
+                return fd;
+            }
+            unlink(filename);
         }
-        unlink(filename);
 
         if ((r = fake_open->fill(cpu_env, fd))) {
             int e = errno;
-- 
2.25.1