RFR: 8263718: unused-result warning happens at os_linux.cpp

[Florian Weimer]

On Fri, 19 Mar 2021 17:33:35 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> I objdump'ed libjvm.so in JDK 16 Linux x64 from jdk.java.net , it also does not seem to expand the stack:
>> 
>> 0000000000bd8500 <thread_native_entry(Thread*)>:
>>   bd8500:       55                      push   %rbp
>>   bd8501:       48 89 e5                mov    %rsp,%rbp
>>   bd8504:       41 56                   push   %r14
>>   bd8506:       41 55                   push   %r13
>>   bd8508:       49 89 fd                mov    %rdi,%r13
>>   bd850b:       41 54                   push   %r12
>>   bd850d:       53                      push   %rbx
>>   bd850e:       e8 ad 1e 1a 00          callq  d7a3c0 <Thread::record_stack_base_and_size()>
>>   bd8513:       e8 08 27 66 ff          callq  23ac20 <getpid at plt>
>>   bd8518:       4c 89 ef                mov    %r13,%rdi
>>   bd851b:       83 05 e6 a3 64 00 01    addl   $0x1,0x64a3e6(%rip)        # 1222908 <thread_native_entry(Thread*)::counter>
>>   bd8522:       e8 39 1e 1a 00          callq  d7a360 <Thread::initialize_thread_current()>
>>   bd8527:       49 8b 9d 70 02 00 00    mov    0x270(%r13),%rbx
>>   bd852e:       31 c0                   xor    %eax,%eax
>> 
>> Result from `getpid()` will be stored into `%RAX`, however it is not used until `xor` at bd852e.
>> And also I could not find out both `alloca()` call and manipulating `%RSP` at here.
>
> Writing to the *end* of the allocated area may do the trick.
> 
> ..Thomas

The use of `getpid` in this code suggests it dates back to LinuxThreads, where the PID differed from thread to thread. On current Linux, `getpid` really returns the PID, so this does nothing to randomize the offset within a single process.

I would expect the system thread library to do this if it is beneficial. glibc replaced `COLORING_INCREMENT` (similar to this `alloca`, I believe) with `MULTI_PAGE_ALIASING` on i386 around 2003. `MULTI_PAGE_ALIASING` is implemented in a completely different way; it tweaks stack sizes to avoid accidental higher-level alignment (above the page level) between different threads.

@hjl-tools Do you think we need anything like this on current CPUs?

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PR: https://git.openjdk.java.net/jdk/pull/3042