[pbs-devel] [PATCH v4 proxmox] Add tempfile() helper function

[Mira Limbeck]

The tempfile() helper function tries to create a temporary file in /tmp
with the O_TMPFILE option. If that fails it falls back to using
mkstemp(). This happens in /tmp/proxmox-<UID> which is either created,
or if it already exists, checked for the right owner and permissions.

As O_TMPFILE was introduced in kernel 3.11 this fallback can help with
CentOS 7 and its 3.10 kernel as well as with WSL (Windows Subsystem for
Linux).

Signed-off-by: Mira Limbeck <m.limbeck at proxmox.com>
---
v4:
 - changed directory from proxmox-backup-<UID> to proxmox-<UID>
 - added check for owner and permissions
v3:
 - O_TMPFILE support is tested on first run of tempfile()
 - EISDIR is handled specifically to test for O_TMPFILE support
 - AtomicBool is used as it provides a safe interface, but 'static mut'
     could also be used
 - mkstemp() now creates the tempfile in a subdirectory called
   proxmox-backup-<UID>

 proxmox/src/tools/fs.rs | 77 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 75 insertions(+), 2 deletions(-)

diff --git a/proxmox/src/tools/fs.rs b/proxmox/src/tools/fs.rs
index b1a95b5..7e13ede 100644
--- a/proxmox/src/tools/fs.rs
+++ b/proxmox/src/tools/fs.rs
@@ -1,17 +1,20 @@
 //! File related utilities such as `replace_file`.
 
 use std::ffi::CStr;
-use std::fs::{File, OpenOptions};
+use std::fs::{DirBuilder, File, OpenOptions};
 use std::io::{self, BufRead, BufReader, Write};
+use std::os::unix::fs::{DirBuilderExt, MetadataExt, OpenOptionsExt, PermissionsExt};
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use std::path::Path;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::time::Duration;
 
 use anyhow::{bail, format_err, Error};
+use lazy_static::lazy_static;
 use nix::errno::Errno;
 use nix::fcntl::OFlag;
 use nix::sys::stat;
-use nix::unistd::{self, Gid, Uid};
+use nix::unistd::{self, geteuid, mkstemp, unlink, Gid, Uid};
 use serde_json::Value;
 
 use crate::sys::error::SysResult;
@@ -518,3 +521,73 @@ pub fn open_file_locked<P: AsRef<Path>>(path: P, timeout: Duration) -> Result<Fi
         Err(err) => bail!("Unable to acquire lock {:?} - {}", path, err),
     }
 }
+
+static O_TMPFILE_SUPPORT: AtomicBool = AtomicBool::new(true);
+lazy_static! {
+    static ref MKSTEMP_PATH: String = {
+        let uid = geteuid();
+        format!("/tmp/proxmox-{}", uid)
+    };
+    static ref MKSTEMP_FILE: String = { format!("{}/tmpfile_XXXXXX", MKSTEMP_PATH.as_str()) };
+}
+
+/// Create a new tempfile by using O_TMPFILE with a fallback to mkstemp() if it fails (e.g. not supported).
+pub fn tempfile() -> Result<File, Error> {
+    if O_TMPFILE_SUPPORT.load(Ordering::Relaxed) {
+        match std::fs::OpenOptions::new()
+            .write(true)
+            .read(true)
+            .custom_flags(libc::O_TMPFILE)
+            .open("/tmp")
+        {
+            Ok(file) => return Ok(file),
+            Err(err) => {
+                let raw_os_error = match err.raw_os_error() {
+                    Some(v) => v,
+                    None => -1,
+                };
+                if raw_os_error == 21 {
+                    O_TMPFILE_SUPPORT.store(false, Ordering::Relaxed);
+                    eprintln!(
+                        "Error creating tempfile: 'EISDIR', falling back to mkstemp() instead",
+                    );
+                } else {
+                    bail!("creating tempfile failed: '{}'", err);
+                }
+            }
+        }
+    }
+
+    match DirBuilder::new().mode(0o700).create(MKSTEMP_PATH.as_str()) {
+        Err(err) => {
+            if err.kind() != std::io::ErrorKind::AlreadyExists {
+                bail!("creating directory failed: '{}'", MKSTEMP_PATH.as_str());
+            } else {
+                // check owner
+                let metadata = std::fs::metadata(MKSTEMP_PATH.as_str())?;
+                if metadata.uid() != geteuid().as_raw() {
+                    bail!(
+                        "directory '{}' has wrong owner: {}",
+                        MKSTEMP_PATH.as_str(),
+                        metadata.uid()
+                    );
+                }
+
+                // check permissions
+                let perm = metadata.permissions();
+                if (perm.mode() & 0o077) != 0 {
+                    bail!(
+                        "directory '{}' already exists with wrong permissions: {:o}",
+                        MKSTEMP_PATH.as_str(),
+                        perm.mode() & 0o777
+                    );
+                }
+            }
+        }
+        _ => {}
+    }
+    let (fd, path) = mkstemp(MKSTEMP_FILE.as_str())?;
+    unlink(path.as_path())?;
+    let file = unsafe { File::from_raw_fd(fd) };
+    Ok(file)
+}
-- 
2.20.1