util-lib: rework /tmp and /var/tmp handling code

Beef up the existing var_tmp() call, rename it to var_tmp_dir() and add a
matching tmp_dir() call (the former looks for the place for /var/tmp, the
latter for /tmp).

Both calls check $TMPDIR, $TEMP, $TMP, following the algorithm Python3 uses.
All dirs are validated before use. secure_getenv() is used in order to limite
exposure in suid binaries.

This also ports a couple of users over to these new APIs.

The var_tmp() return parameter is changed from an allocated buffer the caller
will own to a const string either pointing into environ[], or into a static
const buffer. Given that environ[] is mostly considered constant (and this is
exposed in the very well-known getenv() call), this should be OK behaviour and
allows us to avoid memory allocations in most cases.

Note that $TMPDIR and friends override both /var/tmp and /tmp usage if set.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/journal/journal-verify.c b/src/journal/journal-verify.c
index f61f158e8a..4105abfccc 100644
--- a/src/journal/journal-verify.c
+++ b/src/journal/journal-verify.c
@@ -826,7 +826,7 @@ int journal_file_verify(
         int data_fd = -1, entry_fd = -1, entry_array_fd = -1;
         unsigned i;
         bool found_last = false;
-        _cleanup_free_ char *tmp_dir = NULL;
+        const char *tmp_dir = NULL;
 
 #ifdef HAVE_GCRYPT
         uint64_t last_tag = 0;
@@ -846,7 +846,7 @@ int journal_file_verify(
         } else if (f->seal)
                 return -ENOKEY;
 
-        r = var_tmp(&tmp_dir);
+        r = var_tmp_dir(&tmp_dir);
         if (r < 0) {
                 log_error_errno(r, "Failed to determine temporary directory: %m");
                 goto fail;