Ruby » Ruby master

Bug #12427

Updated by nobu (Nobuyoshi Nakada) over 8 years ago

My gem (msgpack.gem) includes C extension with following pseudo code. 
 This code is working well with released ruby versions. But it caused SEGV with ruby 2.4.0-dev trunk 55091. 

 ```C ``` 
 static VALUE Fixnum_to_msgpack(int argc, VALUE* argv, VALUE self) 
 { 
     long v = FIX2LONG(self); 
     printf("%ld", v);    // work with v 
 } 

 static VALUE Bignum_to_msgpack(int argc, VALUE* argv, VALUE self) 
 { 
     if (RBIGNUM_POSITIVE_P(self)) { 
         uint64_t positive = rb_big2ull(self); 
         printf("%llu", positive);    // work with positive 
     } else { 
         int64_t negative = rb_big2ll(self); 
         printf("%lld", negative);    // work with negative 
     } 
 } 

 void Init_msgpack(void) 
 { 
     rb_define_method(rb_cFixnum, "to_msgpack", Fixnum_to_msgpack, -1); 
     rb_define_method(rb_cBignum, "to_msgpack", Bignum_to_msgpack, -1); 
 } 
 ``` 

 Apparently, since Feature #12005 (https://bugs.ruby-lang.org/issues/12005), both `rb_cFixnum` rb_cFixnum and `rb_cBignum` rb_cBignum are reference to `rb_cInteger` (`rb_cFixnum rb_cInteger (rb_cFixnum == rb_cBignum`). rb_cBignum). Therefore, the later `rb_define_method` rb_define_method call on `rb_cBignum` rb_cBignum overwrites the method with same name on `rb_cFixnum`. rb_cFixnum. So if to_msgpack method is called against an integer in the range of fixnum, `Bignum_to_msgpack` Bignum_to_msgpack function is called against a fixnum object. Then, `RBIGNUM_POSITIVE_P` RBIGNUM_POSITIVE_P is called against a fixnum object. However, because `RBIGNUM_POSITIVE_P` RBIGNUM_POSITIVE_P expects bignum object strictly despite of underlaying unified class definition with fixnum, it causes SEGV. This issue happens differently if the order of `rb_define_method` rb_define_method against `rb_cFixnum` rb_cFixnum and `rb_cBignum` rb_cBignum is opposite. 

 This test code reproduces the problem: 
 https://github.com/msgpack/msgpack-ruby/pull/115/commits/26183b718963f882309d52c167dc866ba5260272 

 I added following fix to the C extension. It succeeded to avoid the problem: 
 https://github.com/msgpack/msgpack-ruby/pull/115/files 

 Concern 1 is that this issue seems a common problem with C extensions and hard to debug unless the author is aware of changes of Feature #12005 including its influence on C API. 
 Concern 2 is that C extensions want to use a macro instead of runtime `if (rb_cFixnum == rb_cBignum)` check to branch code at compile time. 
 I hope that documents or release note of ruby 2.4 includes mention for those concerns and guidelines for their solution. 

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