ODP: Implementation of IO with Panama

[Radosław Smogura]

Hi Maurizio,

I think I know what's happening there - however I would like to find this in some documentation.

The link you shard shown that there's a call to __erno_loation and than move to/from address from rax so result from this call.

But I did objdump and on my Linux the errno is symbol in .tbss section (thread local one),

[ /usr/include ]
radek at radek-ubuntu # objdump -T /usr/lib/x86_64-linux-gnu/libc-2.32.so |grep errno
0000000000000010 g    D  .tbss  0000000000000004  GLIBC_PRIVATE errno
0000000000029030 g    DF .text  0000000000000015  GLIBC_2.2.5 __errno_location
00000000001498d0 g    DF .text  000000000000005f (GLIBC_2.2.5) clnt_sperrno
0000000000149930 g    DF .text  0000000000000084 (GLIBC_2.2.5) clnt_perrno
000000000000006c g    D  .tbss  0000000000000004  GLIBC_PRIVATE __h_errno
0000000000129b60 g    DF .text  0000000000000015  GLIBC_2.2.5 __h_errno_location

D - dynamic or debugging symbol
F - function
g - global

I don't know internals of thread-locals and  dynamic symbols (I only would guess the  first is implemented by kernel by mapping separate page for every thread), and I know linking sometimes can be more complicated (i.e. gcc has multiversioning - something like bootstrap methods).

However I've found this
https://lwn.net/Articles/5851/ (probably outdated)

I think that only manipulation of page map, could make this working.

Definitely it's worth of checking this on BSD and OSX, as well checking what kind of approach should be used, and how it can be determined on runtime, but it looks like errno_location looks like something which should be used.

From the other hand, I work on improving allocator, and I already see some good results, I'll send updates later.

Kind regards,
Rado
[announce, patch] Thread-Local Storage (TLS) support for Linux, 2.5.28 [LWN.net]<https://lwn.net/Articles/5851/>
From:: Ingo Molnar <mingo at elte.hu> To:: linux-kernel at vger.kernel.org: Subject: [announce, patch] Thread-Local Storage (TLS) support for Linux, 2.5.28: Date:
lwn.net





________________________________
Od: Maurizio Cimadamore <maurizio.cimadamore at oracle.com>
Wysłane: piątek, 16 kwietnia 2021 16:30
Do: Radosław Smogura <mail at smogura.eu>
DW: panama-dev at openjdk.java.net <panama-dev at openjdk.java.net>
Temat: Re: Implementation of IO with Panama


On 16/04/2021 15:10, Radosław Smogura wrote:
> I could grab address for it just looking for symbol from CLinker.
>
> But, AFIK, errno is thread local variable so maybe I should double check how this should be handled.


Interesting reading from man page:

```
errno  is  defined  by  the ISO C standard to be a modifiable lvalue of
        type int, and must not be explicitly declared; errno may be  a
macro.
        errno  is  thread-local;  setting  it in one thread does not
affect its
        value in any other thread.
```

this is subtle - it's an lvalue (e.g. can used e.g. for assignment) but
is not explicitly declared - meaning it's not a variable in the proper
sense.

At this point I'm not sure that linking to it gives the expected results.

In fact, firing up Godbolt shows that gcc uses __errno_location under
the hood:

https://godbolt.org/z/rcMPTKxva

Maurizio