Fix format-string use of __FILE__/__func__ and name_for_status lower bound

Two hardening fixes flagged by the earlier review:

1. FAIL passed __FILE__ and __func__ directly as the snprintf format string.
   __FILE__ expands to a string literal that could contain a '%' (a build path
   under a directory with a percent sign), and __func__ is not a literal at all;
   either way snprintf would read nonexistent varargs. Pass them as "%s"
   arguments instead.

2. akerr_name_for_status guarded the upper bound but not the lower one, so a
   negative status indexed __AKERR_ERROR_NAMES[negative] -- an out-of-bounds
   read, or an out-of-bounds write when a name was supplied. Reject status < 0.

Regression tests err_format_string (uses #line to put a conversion specifier in
__FILE__) and err_name_bounds fail against the old code (verified) and pass now.
Full suite: 23/23, no warnings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -69,6 +69,8 @@ set(AKERR_TESTS
     err_maxval
     err_refcount_double_fail
     err_stacktrace_bounds
+    err_name_bounds
+    err_format_string
 )
 
 set(AKERR_WILL_FAIL_TESTS

include/akerror.tmpl.h

@@ -190,8 +190,8 @@ void akerr_init_errno(void);
 #define FAIL(__err_context, __err, __message, ...)			\
     ENSURE_ERROR_READY(__err_context);					\
     __err_context->status = __err;					\
-    snprintf((char *)__err_context->fname, AKERR_MAX_ERROR_FNAME_LENGTH, __FILE__); \
-    snprintf((char *)__err_context->function, AKERR_MAX_ERROR_FUNCTION_LENGTH, __func__); \
+    snprintf((char *)__err_context->fname, AKERR_MAX_ERROR_FNAME_LENGTH, "%s", __FILE__); \
+    snprintf((char *)__err_context->function, AKERR_MAX_ERROR_FUNCTION_LENGTH, "%s", __func__); \
     __err_context->lineno = __LINE__;					\
     snprintf((char *)__err_context->message, AKERR_MAX_ERROR_CONTEXT_STRING_LENGTH, __message, ## __VA_ARGS__); \
     AKERR_STACKTRACE_APPEND(__err_context, "%s:%s:%d: %d (%s) : %s\n", (char *)__err_context->fname, (char *)__err_context->function, __err_context->lineno, __err_context->status, akerr_name_for_status(__err_context->status, NULL), (__err_context->message == NULL ? "" : __err_context->message));

src/error.c

@@ -118,7 +118,7 @@ akerr_ErrorContext *akerr_release_error(akerr_ErrorContext *err)
 // Call with name = NULL to retrieve a status.
 char *akerr_name_for_status(int status, char *name)
 {
-    if ( status > AKERR_MAX_ERR_VALUE ) {
+    if ( status < 0 || status > AKERR_MAX_ERR_VALUE ) {
 	return "Unknown Error";
     }
     if ( name != NULL ) {

tests/err_format_string.c

@@ -0,0 +1,34 @@
+#include "akerror.h"
+#include "err_capture.h"
+#include <string.h>
+
+/*
+ * FAIL records the source file and function names with snprintf. Those names
+ * must be passed as %s ARGUMENTS, not used as the format string -- otherwise a
+ * path containing a printf conversion (say a build directory with a '%') is
+ * interpreted as a format and reads nonexistent varargs (undefined behavior).
+ *
+ * #line lets us make __FILE__ contain a conversion specifier; the stored name
+ * must come back verbatim.
+ */
+
+#line 1 "pct%dname.c"
+akerr_ErrorContext *raise_with_percent_in_filename(void)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, AKERR_VALUE, "boom");
+}
+#line 22 "tests/err_format_string.c"
+
+int main(void)
+{
+    akerr_init();
+
+    akerr_ErrorContext *e = raise_with_percent_in_filename();
+    AKERR_CHECK(e != NULL);
+    AKERR_CHECK(strcmp(e->fname, "pct%dname.c") == 0);
+
+    e = akerr_release_error(e);
+    fprintf(stderr, "err_format_string ok\n");
+    return 0;
+}

tests/err_name_bounds.c

@@ -0,0 +1,31 @@
+#include "akerror.h"
+#include "err_capture.h"
+#include <string.h>
+
+/*
+ * akerr_name_for_status must reject out-of-range status codes at BOTH ends.
+ * It already guarded the upper bound, but a negative status indexed
+ * __AKERR_ERROR_NAMES[negative] -- an out-of-bounds read (and an out-of-bounds
+ * write when a name is supplied). Every out-of-range status must return the
+ * "Unknown Error" sentinel instead.
+ */
+
+int main(void)
+{
+    akerr_init();
+
+    /* Below range. */
+    AKERR_CHECK(strcmp(akerr_name_for_status(-1, NULL), "Unknown Error") == 0);
+    AKERR_CHECK(strcmp(akerr_name_for_status(-9999, NULL), "Unknown Error") == 0);
+
+    /* Above range (already handled; kept as a guard against regressions). */
+    AKERR_CHECK(strcmp(akerr_name_for_status(AKERR_MAX_ERR_VALUE + 1, NULL),
+		       "Unknown Error") == 0);
+
+    /* A valid code must still resolve to its real name. */
+    AKERR_CHECK(strcmp(akerr_name_for_status(AKERR_NULLPOINTER, NULL),
+		       "Null Pointer Error") == 0);
+
+    fprintf(stderr, "err_name_bounds ok\n");
+    return 0;
+}