Assert sanity of name+len arguments to allocmy()

[leonerd]

Recent discussons on github 1 found a bug when calling this function as

allocmy("", 0, 0)

This ought not be allowed. The length must be at least 2, because the function checks the first two characters of name.

• This set of changes does not require a perldelta entry.

[mauke]

Alternatively:

diff --cc op.c
index 6121c7c4f6,6121c7c4f6..5ce0c32181
--- op.c
+++ op.c
@@@ -771,9 -771,9 +771,11 @@@ Perl_allocmy(pTHX_ const char *const na
          croak("panic: allocmy illegal flag bits 0x%" UVxf,
                     (UV)flags);
  
--    is_idfirst = flags & SVf_UTF8
--        ? isIDFIRST_utf8_safe((U8*)name + 1, name + len)
--        : isIDFIRST_A(name[1]);
++    is_idfirst = len >= 2 && (
++        flags & SVf_UTF8
++            ? isIDFIRST_utf8_safe((U8*)name + 1, name + len)
++            : isIDFIRST_A(name[1])
++    );
  
      /* $_, @_, etc. */
      is_default = len == 2 && name[1] == '_';
@@@ -784,9 -784,9 +786,12 @@@
                PL_parser->in_my == KEY_sigvar ? "subroutine signature" :
                PL_parser->in_my == KEY_state  ? "\"state\""     : "\"my\"";
  
--        if (!(flags & SVf_UTF8 && UTF8_IS_START(name[1]))
--         && isASCII(name[1])
--         && (!isPRINT(name[1]) || memCHRs("\t\n\r\f", name[1]))) {
++        if (
++            len >= 2
++            && !(flags & SVf_UTF8 && UTF8_IS_START(name[1]))
++            && isASCII(name[1])
++            && (!isPRINT(name[1]) || memCHRs("\t\n\r\f", name[1]))
++        ) {
              /* diag_listed_as: Can't use global %s in %s */
              yyerror(form("Can't use global %c^%c%.*s in %s",
                                name[0], toCTRL(name[1]),

[leonerd]

I thought about fixing it to allow it, but the only point of allocmy() is that it applies sanity checking of the name as you'd want from the parser, and applies some flags (again as you'd want from the parser). If you were passing an empty name you really wanted to just call pad_add_name_pvn directly.

[mauke]

Yeah, reasonable.