Re: [PATCH] io: Fix record locking contants for powerpc64 with __USE_FILE_OFFSET64

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 29/08/23 13:30, Aurelien Jarno wrote:
> On 2023-08-29 10:20, Adhemerval Zanella Netto wrote:
>>
>>
>> On 29/08/23 04:59, Florian Weimer via Libc-alpha wrote:
>>> * Aurelien Jarno:
>>>
>>>> diff --git a/sysdeps/unix/sysv/linux/powerpc/bits/fcntl.h b/sysdeps/unix/sysv/linux/powerpc/bits/fcntl.h
>>>> index f7615a447e..d8a291a331 100644
>>>> --- a/sysdeps/unix/sysv/linux/powerpc/bits/fcntl.h
>>>> +++ b/sysdeps/unix/sysv/linux/powerpc/bits/fcntl.h
>>>> @@ -33,7 +33,7 @@
>>>>  # define __O_LARGEFILE	0200000
>>>>  #endif
>>>>  
>>>> -#if __WORDSIZE == 64
>>>> +#if __WORDSIZE == 64 && !defined __USE_FILE_OFFSET64
>>>>  # define F_GETLK	5
>>>>  # define F_SETLK	6
>>>>  # define F_SETLKW	7
>>>
>>> I find this puzzling.  Why would __USE_FILE_OFFSET64 have an effect if
>>> __WORDSIZE is 64?
>>
>> This is a historical artifact from powerpc64. Instead of following other 64-bit
>> architectures and define F_GETLK the same whether _FILE_OFFSET_BITS is defined,
>> the port used powerpc definitiosn that required different values to support LFS.
>>
>> This patch is not wrong, but at same time not really required.  The powercp64
>> fcntl will handle F_GETLK/F_SETLK/F_SETLKW with the historical values with
>> the FCNTL_ADJUST_CMD macro, so old binaries will continue to work as expected.
> 
> It does break some binaries, for instance File-FcntlLock [1] with the
> following scenario:
> - perl is built against glibc 2.37
> - system is upgraded to glibc 2.38
> - File-FcntlLock is built against glibc 2.38: it does not work as the
>   value of F_GETLK has changed from 12 to 5 [2].
> 
> In short these constants are not exclusively used by the glibc. They
> might be referenced in some libraries, and changing their values break
> things.
> 
> [1] https://metacpan.org/dist/File-FcntlLock
> [2] https://metacpan.org/release/JTT/File-FcntlLock-0.22/source/FcntlLock.xs#L74
> 

Sigh, you are correct. And it is also means that default and LFS objects are 
essentially incompatible on powerpc64 in this regard for this very reason, but 
I agree that we should keep the compatibility.

Could add a LFS test, just to make sure the lock constants are correct:

diff --git a/io/Makefile b/io/Makefile
index 6ccc0e8691..8a3c83a3bb 100644
--- a/io/Makefile
+++ b/io/Makefile
@@ -192,6 +192,7 @@ tests := \
   tst-fchownat \
   tst-fcntl \
   tst-fcntl-lock \
+  tst-fcntl-lock-lfs \
   tst-fstatat \
   tst-fts \
   tst-fts-lfs \
diff --git a/io/tst-fcntl-lock-lfs.c b/io/tst-fcntl-lock-lfs.c
new file mode 100644
index 0000000000..f2a909fb02
--- /dev/null
+++ b/io/tst-fcntl-lock-lfs.c
@@ -0,0 +1,2 @@
+#define _FILE_OFFSET_BITS 64
+#include <io/tst-fcntl-lock.c>

It won't really trigger this issue, but it a extra sanity test.