[10] RFR(S) 8183262: noexecstack check in os::dll_load on Linux is too expensive

[Vladimir Kozlov]

Thank you, Thomas

I thought about that but choose to do this since I will have to 
duplicate checks done in constructor.

But starting looking on it I realized that my fix is incorrect because 
load_tables() reads file header which is used by 
specifies_noexecstack(). It is mess - os::dll_load() also reads header 
later.

Anyway, to make my life easy I will duplicate header read code in 
specifies_noexecstack().

I will send updated webrev after I test it.

Thanks,
Vladimir

On 6/30/17 12:44 PM, Thomas Stüfe wrote:
> Hi Vladimir,
> 
> This seems fine, but I am a bit concerned that this misuses an
> inconsistency in the API, namely the fact that specifies_noexecstack()
> behaves differently than all other functions (re-read the file each time it
> is called instead of using cached information). In order to understand the
> "load_tables" argument in the constructor, one has to understand that odd
> design, and know that load_tables=false means the only method which can be
> used is specifies_noexecstack().
> 
> As a proposal, could you instead make specifies_noexecstack() a static
> function in ElfFile, such that:
> 
> static bool ElfFile::specifies_noexecstack(const char* filename);
> 
> Then it would be immediately clear that this function is to be used without
> constructing the ElfFile object.
> 
> Kind Regards, Thomas
> 
> On Fri, Jun 30, 2017 at 7:07 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com
>> wrote:
> 
>> webrev: http://cr.openjdk.java.net/~kvn/8183262/webrev/
>> https://bugs.openjdk.java.net/browse/JDK-8183262
>>
>> To load AOT library we use os::dll_load() method provided on all platforms.
>>
>> During investigation why AOT library load takes so long on Linux I found
>> that it spend most of time in ElfFile::load_tables() method which looks for
>> string tables in library. It is called from ElfFile() constructor used in
>> os::dll_load() just to check if library is built with noexecstack:
>>
>>   if (os::uses_stack_guard_pages() && !os::Linux::_stack_is_executable) {
>>     ElfFile ef(filename);
>>     if (!ef.specifies_noexecstack()) {
>>
>> There is no need for string tables load in such case. Avoiding loads helps
>> a lot:
>>
>> without fix:
>>
>> [0.047s][info][aot,startuptime] AOT initialization, 0.0424919 secs
>>
>> with fix:
>>
>> [0.002s][info][aot,startuptime] AOT initialization, 0.0002087 secs
>>
>>
>> Note, JDK 9 is not affected because AOT uses system dlopen() since only
>> Linux is supported.
>>
>> Thanks,
>> Vladimir
>>