llvm dev - Dec 2014 - [LLVMdev] [lld] Handling multiple -init/-fini command line options

Hi,

The LLD linker in gnu flavor mode accepts multiple -init/-fini command
line options. For _all_ symbols specified by these options the linker
creates appropriate entries in the .init_array/.fini_array sections.
But it looks like LD and Gold linkers do not support this feature and
take in account only the last -init/-fini options.

% cat foo.c
int a = 0;
void foo() { a += 1; }
void bar() { a += 2; }

% cat main.c
extern int a;
int printf(const char *, ...);
int main() { printf("a: %d\n", a); }

% gcc -fPIC -shared -o libfoo.so -Wl,-init,foo -Wl,-init,bar foo.c
% gcc main.c -L. -lfoo -Wl,-rpath,.
% ./a.out
2

% gcc -fPIC -shared -o libfoo.so -Wl,-init,bar -Wl,-init,foo foo.c
% ./a.out
1

What is the reason of this incompatibility? The question is caused by
attempt to support DT_INIT/DT_FINI dynamic table tags. The table can
contain no more than one DT_INIT/DT_FINI tags. The LD and Gold linker
look up either default symbols (_init/_fini) or symbols specified by
the -init/-fini options and put their values to the tags.

-- 
Simon Atanasyan

I don't know whether or not it's intended, but it seems like a subtle
but
unnecessary incompatibility. I'd vote for making it compatible with GNU
unless there's a real reason to not do so.

On Mon, Dec 8, 2014 at 3:22 PM, Simon Atanasyan <simon at atanasyan.com>
wrote:
> Hi,
>
> The LLD linker in gnu flavor mode accepts multiple -init/-fini command
> line options. For _all_ symbols specified by these options the linker
> creates appropriate entries in the .init_array/.fini_array sections.
> But it looks like LD and Gold linkers do not support this feature and
> take in account only the last -init/-fini options.
>
> % cat foo.c
> int a = 0;
> void foo() { a += 1; }
> void bar() { a += 2; }
>
> % cat main.c
> extern int a;
> int printf(const char *, ...);
> int main() { printf("a: %d\n", a); }
>
> % gcc -fPIC -shared -o libfoo.so -Wl,-init,foo -Wl,-init,bar foo.c
> % gcc main.c -L. -lfoo -Wl,-rpath,.
> % ./a.out
> 2
>
> % gcc -fPIC -shared -o libfoo.so -Wl,-init,bar -Wl,-init,foo foo.c
> % ./a.out
> 1
>
> What is the reason of this incompatibility? The question is caused by
> attempt to support DT_INIT/DT_FINI dynamic table tags. The table can
> contain no more than one DT_INIT/DT_FINI tags. The LD and Gold linker
> look up either default symbols (_init/_fini) or symbols specified by
> the -init/-fini options and put their values to the tags.
>
> --
> Simon Atanasyan
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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This is a bug with the Gnu linker, IMO. There is also some behavioral 
difference for static executables when the -init/-fini options are used too.

The DT_INIT/DT_FINI correspond to one initializer function,where as 
DT_INIT_ARRAY/DT_FINI_ARRAY is used when there is more than one 
initalizer function/finalizer function respectively.

You could ask this same question in the binutils forums, but I think we 
should use DT_INIT_ARRAY/DT_FINI_ARRAY if more than one init/fini 
function is supplied by the user.

Shankar Easwaran

On 12/8/2014 12:22 AM, Simon Atanasyan wrote:
> Hi,
>
> The LLD linker in gnu flavor mode accepts multiple -init/-fini command
> line options. For _all_ symbols specified by these options the linker
> creates appropriate entries in the .init_array/.fini_array sections.
> But it looks like LD and Gold linkers do not support this feature and
> take in account only the last -init/-fini options.
>
> % cat foo.c
> int a = 0;
> void foo() { a += 1; }
> void bar() { a += 2; }
>
> % cat main.c
> extern int a;
> int printf(const char *, ...);
> int main() { printf("a: %d\n", a); }
>
> % gcc -fPIC -shared -o libfoo.so -Wl,-init,foo -Wl,-init,bar foo.c
> % gcc main.c -L. -lfoo -Wl,-rpath,.
> % ./a.out
> 2
>
> % gcc -fPIC -shared -o libfoo.so -Wl,-init,bar -Wl,-init,foo foo.c
> % ./a.out
> 1
>
> What is the reason of this incompatibility? The question is caused by
> attempt to support DT_INIT/DT_FINI dynamic table tags. The table can
> contain no more than one DT_INIT/DT_FINI tags. The LD and Gold linker
> look up either default symbols (_init/_fini) or symbols specified by
> the -init/-fini options and put their values to the tags.
>

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the
Linux Foundation

On 8 December 2014 at 16:21, Shankar Easwaran <shankare at codeaurora.org>
wrote:
> This is a bug with the Gnu linker, IMO. There is also some behavioral
> difference for static executables when the -init/-fini options are used
too.
>
> The DT_INIT/DT_FINI correspond to one initializer function,where as
> DT_INIT_ARRAY/DT_FINI_ARRAY is used when there is more than one initalizer
> function/finalizer function respectively.
>
> You could ask this same question in the binutils forums, but I think we
> should use DT_INIT_ARRAY/DT_FINI_ARRAY if more than one init/fini function
> is supplied by the user.
One reason why this behaviour is useful in the GNU world is if the
compiler adds an -init flag then the user can add
CFLAGS="-Wl,-init,foo" and override what the compiler passes without
causing an error. How useful this is in this particular case I am not
sure but it holds true for some linker options.
> On 12/8/2014 12:22 AM, Simon Atanasyan wrote:
>>
>> Hi,
>>
>> The LLD linker in gnu flavor mode accepts multiple -init/-fini command
>> line options. For _all_ symbols specified by these options the linker
>> creates appropriate entries in the .init_array/.fini_array sections.
>> But it looks like LD and Gold linkers do not support this feature and
>> take in account only the last -init/-fini options.
>>
>> % cat foo.c
>> int a = 0;
>> void foo() { a += 1; }
>> void bar() { a += 2; }
>>
>> % cat main.c
>> extern int a;
>> int printf(const char *, ...);
>> int main() { printf("a: %d\n", a); }
>>
>> % gcc -fPIC -shared -o libfoo.so -Wl,-init,foo -Wl,-init,bar foo.c
>> % gcc main.c -L. -lfoo -Wl,-rpath,.
>> % ./a.out
>> 2
>>
>> % gcc -fPIC -shared -o libfoo.so -Wl,-init,bar -Wl,-init,foo foo.c
>> % ./a.out
>> 1
>>
>> What is the reason of this incompatibility? The question is caused by
>> attempt to support DT_INIT/DT_FINI dynamic table tags. The table can
>> contain no more than one DT_INIT/DT_FINI tags. The LD and Gold linker
>> look up either default symbols (_init/_fini) or symbols specified by
>> the -init/-fini options and put their values to the tags.
>>
>
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
by
> the Linux Foundation
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


-- 
Will Newton
Toolchain Working Group, Linaro

On Mon, Dec 08, 2014 at 10:21:49AM -0600, Shankar Easwaran
wrote:
> The DT_INIT/DT_FINI correspond to one initializer function,where as
> DT_INIT_ARRAY/DT_FINI_ARRAY is used when there is more than one
> initalizer function/finalizer function respectively.
This is not true. The difference is that the DT_INIT / DT_FINI function
is responsible for calling into the .init / .fini block and for
DT_INIT_ARRAY / DT_FINI_ARRAY, the block is explicitly defined as a
list of pointers and it is the responsibility of either the CRT logic or
the dynamic linker to call them.

That said, I don't think more than one DT_INIT / DT_FINI entry should be
allowed as the latter entry will overwrite the earlier ones.

Joerg

On Sun, Dec 7, 2014 at 10:35 PM, Rui Ueyama <ruiu at google.com>
wrote:
> I don't know whether or not it's intended, but it seems like a
subtle but
> unnecessary incompatibility. I'd vote for making it compatible with GNU
> unless there's a real reason to not do so.
I agree. We should do whatever GNU ld does here.

- Michael Spencer
>
> On Mon, Dec 8, 2014 at 3:22 PM, Simon Atanasyan <simon at
atanasyan.com> wrote:
>>
>> Hi,
>>
>> The LLD linker in gnu flavor mode accepts multiple -init/-fini command
>> line options. For _all_ symbols specified by these options the linker
>> creates appropriate entries in the .init_array/.fini_array sections.
>> But it looks like LD and Gold linkers do not support this feature and
>> take in account only the last -init/-fini options.
>>
>> % cat foo.c
>> int a = 0;
>> void foo() { a += 1; }
>> void bar() { a += 2; }
>>
>> % cat main.c
>> extern int a;
>> int printf(const char *, ...);
>> int main() { printf("a: %d\n", a); }
>>
>> % gcc -fPIC -shared -o libfoo.so -Wl,-init,foo -Wl,-init,bar foo.c
>> % gcc main.c -L. -lfoo -Wl,-rpath,.
>> % ./a.out
>> 2
>>
>> % gcc -fPIC -shared -o libfoo.so -Wl,-init,bar -Wl,-init,foo foo.c
>> % ./a.out
>> 1
>>
>> What is the reason of this incompatibility? The question is caused by
>> attempt to support DT_INIT/DT_FINI dynamic table tags. The table can
>> contain no more than one DT_INIT/DT_FINI tags. The LD and Gold linker
>> look up either default symbols (_init/_fini) or symbols specified by
>> the -init/-fini options and put their values to the tags.
>>
>> --
>> Simon Atanasyan
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>