stdlib: Reject negative precision in io_lib:format

A negative precision passed via ~.* (e.g. io_lib:format("~.*c", [-1, $a]))
would cause infinite recursion in chars/2 since -1 bsr 1 = -1.
Other controls (~.*f, ~.*s, etc.) would crash with function_clause.

Fix by validating precision >= 0 in precision/2, raising badarg
consistently for all controls. This matches C printf semantics
where negative precision via * is defined behavior (treated as
omitted), but Erlang chooses to reject it explicitly.

Author: dgud

lib/stdlib/src/io_lib_format.erl

@@ -262,7 +262,12 @@ field_width(F, Fmt, Args) when F >= 0 ->
     {F,right,Fmt,Args}.
 
 precision([$.|Fmt], Args) ->
-    field_value(Fmt, Args);
+    case field_value(Fmt, Args) of
+        {P, _, _} when is_integer(P), P < 0 ->
+            error(badarg);
+        Result ->
+            Result
+    end;
 precision(Fmt, Args) ->
     {none,Fmt,Args}.
 

lib/stdlib/test/io_SUITE.erl

@@ -40,7 +40,7 @@
          native_records/1, cover_fread/1,
          format_w_empty_map/1, format_w_limited/1,
          write_record_maps_order/1, write_record_latin1_encoding/1,
-         indentation_tab/1]).
+         indentation_tab/1, badarg/1]).
 
 -export([pretty/2, trf/3, rfd/2]).
 
@@ -80,7 +80,7 @@ all() ->
      native_records,
      format_w_empty_map, format_w_limited,
      write_record_maps_order, write_record_latin1_encoding,
-     indentation_tab,
+     indentation_tab, badarg,
      cover_fread].
 
 %% Error cases for output.
@@ -3544,6 +3544,23 @@ indentation_tab(_Config) ->
     10 = io_lib_format:indentation("ab\ncd\tef", 0),
     ok.
 
+%% Test that negative precision via * raises badarg.
+badarg(_Config) ->
+    bad_io_lib_format("~.*c", [-1, $a]),
+    bad_io_lib_format("~.*s", [-1, "hello"]),
+    bad_io_lib_format("~.*f", [-1, 1.5]),
+    bad_io_lib_format("~.*e", [-1, 1.5]),
+    bad_io_lib_format("~.*g", [-1, 1.5]),
+    bad_io_lib_format("~.*b", [-1, 42]),
+    bad_io_lib_format("~.*B", [-1, 42]),
+    bad_io_lib_format("~.*x", [-1, 42, "0x"]),
+    bad_io_lib_format("~.*X", [-1, 42, "0x"]),
+    bad_io_lib_format("~.*+", [-1, 42]),
+    bad_io_lib_format("~.*#", [-1, 42]),
+    bad_io_lib_format("~.*w", [-1, foo]),
+    bad_io_lib_format("~.*p", [-1, foo]),
+    ok.
+
 fread_bad(Format, String) ->
     {error,{fread,Hint}} = io_lib:fread(Format, String),
     Hint.