RFR: 8283059: Uninitialized warning in check_code.c with GCC 11.2

[Kim Barrett]

On Thu, 17 Mar 2022 08:08:07 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Background, from JBS:
>> 
>> src/java.base/share/native/libverify/check_code.c: In function 'read_all_code': 
>> src/java.base/share/native/libverify/check_code.c:942:5: error: 'lengths' may be used uninitialized [-Werror=maybe-uninitialized] 
>>   942 | check_and_push(context, lengths, VM_MALLOC_BLK); 
>>       | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
>> src/java.base/share/native/libverify/check_code.c:4145:13: note: by argument 2 of type 'const void *' to 'check_and_push' declared here 
>>  4145 | static void check_and_push(context_type *context, const void *ptr, int kind) 
>>       | ^~~~~~~~~~~~~~ 
>> 
>> 
>> Because the second argument of check_and_push is "const void*" GCC assumes that the malloc:ed data, which has not yet been initialized, will not be/can not be modified later which in turn suggests it may be used without ever being initialized. 
>> 
>> The same general issue was addressed in [JDK-8266168](https://bugs.openjdk.java.net/browse/JDK-8266168), presumably for GCC 11.1.
>> 
>> 
>> Details:
>> 
>> Instead of sprinkling more calloc calls around or using pragmas/gcc attributes I chose to change the check_and_push function to take a (non-const) void* argument, and provide a new wrapper function `check_and_push_const` which handles the const argument case. For the (non-const) VM_MALLOC_BKP that means the pointer never needs to go through a const conversion.
>> 
>> To avoid having multiple ways of solving the same problem I also chose to revert the change made in JDK-8266168, reverting the calloc back to a malloc call.
>> 
>> Testing:
>> 
>> tier1 + builds-tier{2,3,4,5}
>
> src/java.base/share/native/libverify/check_code.c line 472:
> 
>> 470: 
>> 471: static void check_and_push(context_type *context, void *ptr, int kind);
>> 472: static void check_and_push_const(context_type *context, const void *ptr, int kind);
> 
> This really needs a comment explaining why two functions are needed.

I think it would be clearer to drop the kind argument and have two functions
check_and_push_vm_string(context_type*, const char*)
check_and_push_malloc_block(context_type*, void*)
with a shared helper that is given the appropriate kind.

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PR: https://git.openjdk.java.net/jdk/pull/7794