Fail on unknown (alphanumerical) specifiers

The code intentionally ignored unknown specifiers, treating them as text. This
needs to change because otherwise we can never add a new specifier in a backwards
compatible way. So just treat an unknown (potential) specifier as an error.

In principle this is a break of backwards compatibility, but the previous
behaviour was pretty much useless, since the expanded value could change every
time we add new specifiers, which we do all the time.

As a compromise for backwards compatibility, only fail on alphanumerical
characters. This should cover the most cases where an unescaped percent
character is used, like size=5% and such, which behave the same as before with
this patch. OTOH, this means that we will not be able to use non-alphanumerical
specifiers without breaking backwards compatibility again. I think that's an
acceptable compromise.

v2:
- add NEWS entry

v3:
- only fail on alphanumerical

1 files changed, 8 insertions, 1 deletions

diff --git a/src/shared/specifier.c b/src/shared/specifier.c
index 9bef890346..9034cc20d7 100644
--- a/src/shared/specifier.c
+++ b/src/shared/specifier.c
@@ -41,6 +41,10 @@
  *
  */
 
+/* Any ASCII character or digit: our pool of potential specifiers,
+ * and "%" used for escaping. */
+#define POSSIBLE_SPECIFIERS ALPHANUMERICAL "%"
+
 int specifier_printf(const char *text, const Specifier table[], void *userdata, char **_ret) {
         char *ret, *t;
         const char *f;
@@ -97,7 +101,10 @@ int specifier_printf(const char *text, const Specifier table[], void *userdata,
 
                                         ret = n;
                                         t = n + j + k;
-                                } else {
+                                } else if (strchr(POSSIBLE_SPECIFIERS, *f))
+                                        /* Oops, an unknown specifier. */
+                                        return -EBADSLT;
+                                else {
                                         *(t++) = '%';
                                         *(t++) = *f;
                                 }