Figuring out start and end of sections

Figuring out start and end of sections

[Felipe Balbi]

Hi all,

is there any way to figure out where a section starts and ends ?

I added a specific section to my program using
__attribute__((section "<section name>")) and now I want to figure out
where that section starts so I can iterate over it and call the function
pointers I'm adding to it.

Do I need a specific linker script to achieve that or does the default
scripts give me possibility to find that out ?

-- 
balbi

Re: Figuring out start and end of sections

[Andrew Haley]

On 04/13/2010 06:40 PM, Felipe Balbi wrote:

> is there any way to figure out where a section starts and ends ?
> 
> I added a specific section to my program using
> __attribute__((section "<section name>")) and now I want to figure out
> where that section starts so I can iterate over it and call the function
> pointers I'm adding to it.

Terminate the list with a null pointer; AFAIK that's what everyone else
does.  To get the start address, you just need to define a global variable
in that section.  Of course this means you have to link everything in the
correct order.

Andrew.

Re: Figurig out start and end of sections

[Felipe Balbi]

Hi Andrew,

for some reason I didn't receive your reply.

>On 04/13/2010 06:40 PM, Felipe Balbi wrote:
>> is there any way to figure out where a section starts and ends ?
>> 
>> I added a specific section to my program using
>> __attribute__((section "<section name>")) and now I want to figure out
>> where that section starts so I can iterate over it and call the
>> function
>> pointers I'm adding to it.
>
>Terminate the list with a null pointer; AFAIK that's what everyone else
>does.  To get the start address, you just need to define a global
>variable
>in that section.  Of course this means you have to link everything in
>the
>correct order.

I think you're talking about something like:

void (*func_ptr)(void)[] __attribute__((section ".my_section")) = {
	func1,
	func2,
	func3,
	NULL,
};

is that right ? (the code above isn't really correct).

I don't really have an array. I'm trying to mimic what the kernel does
with the modules, but then again, the kernel declares the constants for
the start and end of initcalls in constants within the ld script.

So what I have right now is:

main.c:

[..]

int __attribute__((section(".init"))) register_struct(struct my_struct *ptr)
{
	[..]
}

[..]

static int call_initcalls(void)
{
	initcall_t	*fn;

	for (fn = __start_initcall; fn; fn++)
		fn();

	return 0;
}

then on another source code, I define an initcall_t.

initcall.c:

[..]

static int __attribute__((section(".init"))) init(void)
{
	return register_struct(ptr);
}

static initcall_t initcall_t_init __attribute__((section(".my.init")))
	__attribute__((__used__)) = init;

then several little "plugins" would define that and all those pointers
are going to the right section. Now how to fetch them and call them ??
currently I'm getting segfault. :-p

-- 
balbi

Re: Figurig out start and end of sections

[Andrew Haley]

On 04/14/2010 06:08 AM, Felipe Balbi wrote:
> Hi Andrew,
> 
> for some reason I didn't receive your reply.
> 
>> On 04/13/2010 06:40 PM, Felipe Balbi wrote:
>>> is there any way to figure out where a section starts and ends ?
>>>
>>> I added a specific section to my program using
>>> __attribute__((section "<section name>")) and now I want to figure out
>>> where that section starts so I can iterate over it and call the
>>> function
>>> pointers I'm adding to it.
>>
>> Terminate the list with a null pointer; AFAIK that's what everyone else
>> does.  To get the start address, you just need to define a global
>> variable
>> in that section.  Of course this means you have to link everything in
>> the
>> correct order.
> 
> I think you're talking about something like:
> 
> void (*func_ptr)(void)[] __attribute__((section ".my_section")) = {
> 	func1,
> 	func2,
> 	func3,
> 	NULL,
> };

No.

void (*func_ptr)(void) __attribute__((section ".my_section") = foo;

void (*func_ptr)(void) __attribute__((section ".my_section") = bar;

etc, etc.  I'm assuming the entries are in different files.
And in the last file,

void *poo __attribute__((section ".my_section") = NULL;

> is that right ? (the code above isn't really correct).
> 
> I don't really have an array. I'm trying to mimic what the kernel does
> with the modules, but then again, the kernel declares the constants for
> the start and end of initcalls in constants within the ld script.
> 
> So what I have right now is:
> 
> main.c:
> 
> [..]
> 
> int __attribute__((section(".init"))) register_struct(struct my_struct *ptr)
> {
> 	[..]
> }
> 
> [..]
> 
> static int call_initcalls(void)
> {
> 	initcall_t	*fn;
> 
> 	for (fn = __start_initcall; fn; fn++)
> 		fn();

I don't understand this.  How can you increment a function pointer?


> 
> 	return 0;
> }
> 
> then on another source code, I define an initcall_t.
> 
> initcall.c:
> 
> [..]
> 
> static int __attribute__((section(".init"))) init(void)
> {
> 	return register_struct(ptr);
> }
> 
> static initcall_t initcall_t_init __attribute__((section(".my.init")))
> 	__attribute__((__used__)) = init;
> 
> then several little "plugins" would define that and all those pointers
> are going to the right section. Now how to fetch them and call them ??
> currently I'm getting segfault. :-p
> 

Re: Figuring out start and end of sections

[Ian Lance Taylor]

Felipe Balbi <me@felipebalbi.com> writes:

> is there any way to figure out where a section starts and ends ?
>
> I added a specific section to my program using
> __attribute__((section "<section name>")) and now I want to figure out
> where that section starts so I can iterate over it and call the function
> pointers I'm adding to it.
>
> Do I need a specific linker script to achieve that or does the default
> scripts give me possibility to find that out ?

If you are using the GNU linker, or gold, and you make the section
name a valid C identifier, then the linker will automatically define
symbols __start_SECNAME and __stop_SECNAME which you can use.

Ian

Re: Figuring out start and end of sections

[Andrew Haley]

On 04/14/2010 01:24 AM, Ian Lance Taylor wrote:
> Felipe Balbi <me@felipebalbi.com> writes:
> 
>> is there any way to figure out where a section starts and ends ?
>>
>> I added a specific section to my program using
>> __attribute__((section "<section name>")) and now I want to figure out
>> where that section starts so I can iterate over it and call the function
>> pointers I'm adding to it.
>>
>> Do I need a specific linker script to achieve that or does the default
>> scripts give me possibility to find that out ?
> 
> If you are using the GNU linker, or gold, and you make the section
> name a valid C identifier, then the linker will automatically define
> symbols __start_SECNAME and __stop_SECNAME which you can use.

Ah, that's a much better idea.  Forget my suggestion.

Andrew.