RFR: 8314114: Fix -Wconversion warnings in os code, primarily linux

[Coleen Phillimore]

On Thu, 10 Aug 2023 18:58:19 GMT, Dean Long <dlong at openjdk.org> wrote:

>> This change fixes various -Wconversion warnings from files in the os directories and related, widening types or adding checked_cast<> where narrowing.
>> Tested with tier1 on linux/macosx/windows and tier1-4 on linux-x64-debug, windows-x64-debug.
>
> src/hotspot/os/aix/attachListener_aix.cpp line 289:
> 
>> 287: 
>> 288:   ssize_t off = 0;
>> 289:   ssize_t left = max_len;
> 
> Doesn't this cause a sign-conversion warning because read expects size_t?

I don't have -Wsign-conversion on, only -Wconversion, so I don't get -Wsign-conversion warnings for hotspot files.  This same change is made to os/linux/attachListener_linux.cpp.  Suggestions how to fix?

> src/hotspot/os/aix/os_aix.cpp line 3026:
> 
>> 3024:   time_t t1 = get_mtime(file1);
>> 3025:   time_t t2 = get_mtime(file2);
>> 3026:   return checked_cast<int>(t1 - t2);
> 
> It seems like checked_cast<> could fail if the file has a bad timestamp far in the future.
> Suggestion:
> 
>   return primitive_compare(t1, t2);

This is a good idea to use primitive_compare.

> src/hotspot/os/linux/os_linux.cpp line 1604:
> 
>> 1602: 
>> 1603:   Elf32_Ehdr elf_head;
>> 1604:   size_t diag_msg_max_length=ebuflen-strlen(ebuf);
> 
> Underflow will result in very large unsigned number.  Do we need helper functions for unsigned math that checks for underflow/overflow?

I could have just checked_cast<int>(strlen(ebuf)); since it's 100% unlikely that the buffer is 2,147,483,647 chars long. I don't understand why strlen returns a size_t tbh.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15229#discussion_r1290627670
PR Review Comment: https://git.openjdk.org/jdk/pull/15229#discussion_r1290629641
PR Review Comment: https://git.openjdk.org/jdk/pull/15229#discussion_r1290644112