[RFC] Add __initbss section

[RFC] Add __initbss section

[Franck Bui-Huu]

Hi,

As discussed previously, it seems a good idea to create a new init
section .init.bss that store uninitialized data used only at boot
time. That way, we can avoid to embed these uninitialized data in the
Linux image since it's totaly useless.

As a good candidate for using this, is tlbex.c. This file allocates a
couple of big arrays that don't need to be part of the init data
section since they're not initialized and they're currently only used
at boot time.

So this patchset does this but the result looks weird: I tried to
apply this patch on top of the patchset:

---8<---

diff --git a/arch/mips/mm/tlbex.c b/arch/mips/mm/tlbex.c
index a61246d..8271eab 100644
--- a/arch/mips/mm/tlbex.c
+++ b/arch/mips/mm/tlbex.c
@@ -743,11 +743,11 @@ il_bgez(u32 **p, struct reloc **r, unsigned int reg, enum label_id l)
  * We deliberately chose a buffer size of 128, so we won't scribble
  * over anything important on overflow before we panic.
  */
-static __initdata u32 tlb_handler[128];
+static __initbss u32 tlb_handler[128];
 
 /* simply assume worst case size for labels and relocs */
-static __initdata struct label labels[128];
-static __initdata struct reloc relocs[128];
+static __initbss struct label labels[128];
+static __initbss struct reloc relocs[128];

--->8---

and the kernel image is bigger after the patch is applied !

$ ls -l vmlinux*
-rwxrwxr-x 1 fbuihuu fbuihuu 2503324 2007-10-11 11:41 vmlinux*
-rwxrwxr-x 1 fbuihuu fbuihuu 2503264 2007-10-11 11:41 vmlinux~old*

Could anybody explain me why ? The time is missing and I probably
couldn't investigate into this until this weekend. 

Also not that with the current patchset applied, there are now 2
segments that need to be loaded, hopefully it won't cause any issues
with any bootloaders out there that would assume that an image has
only one segment...

Other question: I noticed that the exit.data section is not
discarded. Could anybody give me the reason why ?

		Franck

[PATCH 1/2] Add .init.bss section

[Franck Bui-Huu]

This patch creates a new init section called .init.bss.

This section is similar to .init.data but doesn't consume
any space in the vmlinux image.

All data marked as part of this section must not be initialized,
of course.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
---
 include/linux/init.h |   24 ++++++++++++++++--------
 1 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/include/linux/init.h b/include/linux/init.h
index 74b1f43..9fda0ec 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -43,6 +43,8 @@
 #define __init		__attribute__ ((__section__ (".init.text"))) __cold
 #define __initdata	__attribute__ ((__section__ (".init.data")))
 #define __exitdata	__attribute__ ((__section__(".exit.data")))
+#define __initbss	__attribute__ ((__section__ (".init.bss")))
+#define __exitbss	__attribute__ ((__section__ (".exit.bss")))
 #define __exit_call	__attribute_used__ __attribute__ ((__section__ (".exitcall.exit")))
 
 /* modpost check for section mismatches during the kernel build.
@@ -257,10 +259,12 @@ void __init parse_early_param(void);
 #define __devexit
 #define __devexitdata
 #else
-#define __devinit __init
-#define __devinitdata __initdata
-#define __devexit __exit
-#define __devexitdata __exitdata
+#define __devinit	__init
+#define __devinitdata	__initdata
+#define __devinitbss	__initbss
+#define __devexit	__exit
+#define __devexitdata	__exitdata
+#define __devexitbss	__exitbss
 #endif
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -270,9 +274,11 @@ void __init parse_early_param(void);
 #define __cpuexitdata
 #else
 #define __cpuinit	__init
-#define __cpuinitdata __initdata
-#define __cpuexit __exit
+#define __cpuinitdata	__initdata
+#define __cpuinitbss	__initbss
+#define __cpuexit	__exit
 #define __cpuexitdata	__exitdata
+#define __cpuexitbss	__exitbss
 #endif
 
 #if defined(CONFIG_MEMORY_HOTPLUG) || defined(CONFIG_ACPI_HOTPLUG_MEMORY) \
@@ -283,9 +289,11 @@ void __init parse_early_param(void);
 #define __memexitdata
 #else
 #define __meminit	__init
-#define __meminitdata __initdata
-#define __memexit __exit
+#define __meminitdata	__initdata
+#define __meminitbss	__meminitbss
+#define __memexit	__exit
 #define __memexitdata	__exitdata
+#define __memexitbss	__exitbss
 #endif
 
 /* Functions marked as __devexit may be discarded at kernel link time, depending
-- 
1.5.3.3

[PATCH 2/2] Add .init.bss section for MIPS

[Franck Bui-Huu]

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
---
 arch/mips/kernel/head.S        |    5 +++++
 arch/mips/kernel/vmlinux.lds.S |    7 ++++++-
 2 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/arch/mips/kernel/head.S b/arch/mips/kernel/head.S
index e46782b..e8245cd 100644
--- a/arch/mips/kernel/head.S
+++ b/arch/mips/kernel/head.S
@@ -183,6 +183,11 @@ NESTED(kernel_entry, 16, sp)			# kernel entry point
 	LONG_S		zero, (t0)
 	bne		t0, t1, 1b
 
+	PTR_LA		a0, _sinitbss
+	PTR_LA		a1, _einitbss
+	PTR_SUBU	a1, a0
+	jal		__bzero
+
 	LONG_S		a0, fw_arg0		# firmware arguments
 	LONG_S		a1, fw_arg1
 	LONG_S		a2, fw_arg2
diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 84f9a4c..30e0d65 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -100,7 +100,7 @@ SECTIONS
 	_edata =  .;			/* End of data section */
 
 	/* will be freed after init */
-	. = ALIGN(_PAGE_SIZE);		/* Init code and data */
+	. = ALIGN(_PAGE_SIZE);		/* Init code, data and bss */
 	__init_begin = .;
 	.init.text : {
 		_sinittext = .;
@@ -110,6 +110,11 @@ SECTIONS
 	.init.data : {
 		*(.init.data)
 	}
+	.init.bss (NOLOAD) : {
+		_sinitbss = .;
+		*(.init.bss)
+		_einitbss = .;
+	}
 	. = ALIGN(16);
 	.init.setup : {
 		__setup_start = .;
-- 
1.5.3.3

Re: [RFC] Add __initbss section

[Ralf Baechle]

On Thu, Oct 11, 2007 at 11:52:30AM +0200, Franck Bui-Huu wrote:

> Other question: I noticed that the exit.data section is not
> discarded. Could anybody give me the reason why ?

.exit.data and .exit.text may reference each other.  __exit functions
generally get compiled into .exit.text but some constructs such as jump
tables for switch() constructs may be compiled into address tables which
gcc unfortunately will put into .rodata, so .rodata will end up
referencing function addresses in .exit.text which makes ld unhappy if
.exit.text was discarded.  So until this is fixed in gcc we can't
discard exit code, unfortunately.

It's actually an issue which doesn't strike very often, so users who are
desparate for shrinking the kernel down could try to undo patchsets:

  6f0b1e5d266fb1d0da019c07b56ccc02c3a4f53a
  ca7402fed2a76cd5a417ac4d375a5539dcb2b6de

and see if they can get away with it.  If the final kernel link succeeds,
the build would be ok.

I think gcc should probably put the jump table into a .subsection if
a section was explicitly requested, at least for non-PIC code.

  Ralf

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Thu, 11 Oct 2007, Franck Bui-Huu wrote:

> and the kernel image is bigger after the patch is applied !
> 
> $ ls -l vmlinux*
> -rwxrwxr-x 1 fbuihuu fbuihuu 2503324 2007-10-11 11:41 vmlinux*
> -rwxrwxr-x 1 fbuihuu fbuihuu 2503264 2007-10-11 11:41 vmlinux~old*
> 
> Could anybody explain me why ? The time is missing and I probably
> couldn't investigate into this until this weekend. 

 I guess for a bss-type section you want to use something like:

	.section .init.bss,"aw",@nobits

> Also not that with the current patchset applied, there are now 2
> segments that need to be loaded, hopefully it won't cause any issues
> with any bootloaders out there that would assume that an image has
> only one segment...

 Well, there should be no need for an extra segment -- just rearrange the 
order of the sections in the linker script appropriately.  You should 
probably add __exitbss for consistency too.  You can make all the three 
sections adjacent so that no separate initialisation is required.

 Another place for optimisation are inline string literals referred to 
from .init.text or .exit.text only, which is a bit more complicated to 
handle, but sounds interesting enough to me I shall give it a try in the 
next few days.  I have an idea how to do it.

  Maciej

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Thu, 11 Oct 2007, Ralf Baechle wrote:

> I think gcc should probably put the jump table into a .subsection if
> a section was explicitly requested, at least for non-PIC code.

 Has anybody filed a bug report?  The GCC maintainers may well not be 
aware of the problem and some arcane ways of use exercised by Linux.

  Maciej

Re: [RFC] Add __initbss section

[Ralf Baechle]

On Thu, Oct 11, 2007 at 02:35:20PM +0100, Maciej W. Rozycki wrote:

> > I think gcc should probably put the jump table into a .subsection if
> > a section was explicitly requested, at least for non-PIC code.
> 
>  Has anybody filed a bug report?  The GCC maintainers may well not be 
> aware of the problem and some arcane ways of use exercised by Linux.

Beofore applying the previously mentioned fixes I spoke to them but they
were not very inclined to consider the gcc behaviour a bug.

  Ralf

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Thu, 11 Oct 2007, Ralf Baechle wrote:

> Beofore applying the previously mentioned fixes I spoke to them but they
> were not very inclined to consider the gcc behaviour a bug.

 This may qualify as the lack of a feature indeed.  So I guess the 
ultimate solution is waiting for somebody to get inclined enough to 
implement it. ;-)

  Maciej

Re: [RFC] Add __initbss section

[Thiemo Seufer]

Franck Bui-Huu wrote:
[snip]
> Also not that with the current patchset applied, there are now 2
> segments that need to be loaded, hopefully it won't cause any issues
> with any bootloaders out there that would assume that an image has
> only one segment...

This breaks at least conversion to ECOFF as used on DECstations.
Srec might also be affected, I'm not sure about that.


Thiemo

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Thu, 11 Oct 2007, Thiemo Seufer wrote:

> > Also not that with the current patchset applied, there are now 2
> > segments that need to be loaded, hopefully it won't cause any issues
> > with any bootloaders out there that would assume that an image has
> > only one segment...
> 
> This breaks at least conversion to ECOFF as used on DECstations.

 Indeed, thanks for pointing it out -- I use ELF over MOP and keep 
forgetting about people preferring TFTP, sigh.  I wonder whether `objcopy' 
might be doing a better job than `elf2ecoff' these days though.  It may be 
worth checking...

 It all boils down to padding out what cannot be expressed in a less 
capable format after all.  That's what I did in `mopd' for ELF support too 
-- holes between segments are transmitted as zeroes.  Though MOP clients 
may actually support discontiguous images as each MOP "downline load" 
packet has its designated memory load address included in the header (it 
just shows how reasonable a bootstrap protocol it is!).

  Maciej

Re: [RFC] Add __initbss section

[Ralf Baechle]

On Thu, Oct 11, 2007 at 05:16:45PM +0100, Maciej W. Rozycki wrote:

> > > Also not that with the current patchset applied, there are now 2
> > > segments that need to be loaded, hopefully it won't cause any issues
> > > with any bootloaders out there that would assume that an image has
> > > only one segment...
> > 
> > This breaks at least conversion to ECOFF as used on DECstations.
> 
>  Indeed, thanks for pointing it out -- I use ELF over MOP and keep 
> forgetting about people preferring TFTP, sigh.  I wonder whether `objcopy' 
> might be doing a better job than `elf2ecoff' these days though.  It may be 
> worth checking...

That would be quite unexpected.  Binutils has severe issues when used
with multiple binary formats.  For the general case fixing and maintaining
that would be of almost nightmarish complexity so years ago I settled for
elf2ecoff and I guess that step also made binutils maintainers alot
happier ;-)

>  It all boils down to padding out what cannot be expressed in a less 
> capable format after all.  That's what I did in `mopd' for ELF support too 
> -- holes between segments are transmitted as zeroes.  Though MOP clients 
> may actually support discontiguous images as each MOP "downline load" 
> packet has its designated memory load address included in the header (it 
> just shows how reasonable a bootstrap protocol it is!).

With ELF dumb more generic protocols like TFTP work great.  Shows what
a great format ELF is ;-)

  Ralf

Re: [RFC] Add __initbss section

[Thiemo Seufer]

Maciej W. Rozycki wrote:
> On Thu, 11 Oct 2007, Thiemo Seufer wrote:
> 
> > > Also not that with the current patchset applied, there are now 2
> > > segments that need to be loaded, hopefully it won't cause any issues
> > > with any bootloaders out there that would assume that an image has
> > > only one segment...
> > 
> > This breaks at least conversion to ECOFF as used on DECstations.
> 
>  Indeed, thanks for pointing it out -- I use ELF over MOP and keep 
> forgetting about people preferring TFTP, sigh.  I wonder whether `objcopy' 
> might be doing a better job than `elf2ecoff' these days though.  It may be 
> worth checking...

The problem seems to be that (DECstation-firmware's view of) ECOFF can
only handle a single .text segment.

>  It all boils down to padding out what cannot be expressed in a less 
> capable format after all.  That's what I did in `mopd' for ELF support too 
> -- holes between segments are transmitted as zeroes.

AFAICS this won't work for interleaved .text / .data / .bss segments.


Thiemo

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Thu, 11 Oct 2007, Ralf Baechle wrote:

> That would be quite unexpected.  Binutils has severe issues when used
> with multiple binary formats.  For the general case fixing and maintaining
> that would be of almost nightmarish complexity so years ago I settled for
> elf2ecoff and I guess that step also made binutils maintainers alot
> happier ;-)

 I have now recalled what the issue is -- `objcopy' is keen to copy the 
file header as well as possible and that includes MIPS-III annotation 
which makes the firmware of R4k-based DECstations unhappy.  They all 
expect MIPS-I binaries.  It's not binutils's fault I am afraid.

> With ELF dumb more generic protocols like TFTP work great.  Shows what
> a great format ELF is ;-)

 Yeah, sure...  Until you discover your ELF parser in the firmware does 
not do this and that and also crashes on yet something else (since when 
has CFE supported ELF64, then?).  You cannot escape such bugs as one with 
ECOFF above for example.  And if your firmware is a binary blob in a 
classic PROM, then you are bust.

 At least an <address,size> pair is dumb enough it is hard to get it wrong 
and `mopd' may be modified however you like to get what you need; the 
daemon itself is not interested in the gory details of the binary being 
sent, so it will accept about anything remotely conforming to the ELF 
format anyway. :-)

  Maciej

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Ralf Baechle wrote:
> On Thu, Oct 11, 2007 at 11:52:30AM +0200, Franck Bui-Huu wrote:
> 
>> Other question: I noticed that the exit.data section is not
>> discarded. Could anybody give me the reason why ?
> 
> .exit.data and .exit.text may reference each other.  __exit functions
> generally get compiled into .exit.text but some constructs such as jump
> tables for switch() constructs may be compiled into address tables which
> gcc unfortunately will put into .rodata, so .rodata will end up
> referencing function addresses in .exit.text which makes ld unhappy if
> .exit.text was discarded.  So until this is fixed in gcc we can't
> discard exit code, unfortunately.
> 

Thanks for the details.

I actually don't see any point to move these tables in .rodata since
they're part of the code...

> It's actually an issue which doesn't strike very often, so users who are
> desparate for shrinking the kernel down could try to undo patchsets:
> 
>   6f0b1e5d266fb1d0da019c07b56ccc02c3a4f53a
>   ca7402fed2a76cd5a417ac4d375a5539dcb2b6de
> 
> and see if they can get away with it.  If the final kernel link succeeds,
> the build would be ok.
> 
> I think gcc should probably put the jump table into a .subsection if
> a section was explicitly requested, at least for non-PIC code.
> 

yes that would be great, and do the same for strings, data and we could
get rid of all __initxxx annotations.

		Franck

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Maciej W. Rozycki wrote:
> On Thu, 11 Oct 2007, Franck Bui-Huu wrote:
> 
>> and the kernel image is bigger after the patch is applied !
>>
>> $ ls -l vmlinux*
>> -rwxrwxr-x 1 fbuihuu fbuihuu 2503324 2007-10-11 11:41 vmlinux*
>> -rwxrwxr-x 1 fbuihuu fbuihuu 2503264 2007-10-11 11:41 vmlinux~old*
>>
>> Could anybody explain me why ? The time is missing and I probably
>> couldn't investigate into this until this weekend. 
> 
>  I guess for a bss-type section you want to use something like:
> 
> 	.section .init.bss,"aw",@nobits
> 

Sorry but I'm missing your point here. This indeed should be added
for assembler code but I don't see how it's related with the kernel
image size difference I was seeing.

> 
>  Well, there should be no need for an extra segment -- just rearrange the 
> order of the sections in the linker script appropriately.  You should 
> probably add __exitbss for consistency too.  You can make all the three 
> sections adjacent so that no separate initialisation is required.
> 

I'll do that.

		Franck

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Thiemo Seufer wrote:
> Franck Bui-Huu wrote:
> [snip]
>> Also not that with the current patchset applied, there are now 2
>> segments that need to be loaded, hopefully it won't cause any issues
>> with any bootloaders out there that would assume that an image has
>> only one segment...
> 
> This breaks at least conversion to ECOFF as used on DECstations.
> Srec might also be affected, I'm not sure about that.
> 

Ok, so I will reorder the things to have only one segment.

Thanks.
		Franck

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Sun, 14 Oct 2007, Franck Bui-Huu wrote:

> >  I guess for a bss-type section you want to use something like:
> > 
> > 	.section .init.bss,"aw",@nobits
> > 
> 
> Sorry but I'm missing your point here. This indeed should be added
> for assembler code but I don't see how it's related with the kernel
> image size difference I was seeing.

 Well, otherwise the section is marked as containing data and therefore 
taking space in the image.  Compare the output of `readelf -S' and/or 
`objdump -h' with and without the directive above.

 What do you mean by "assembler code" in this context, BTW?  Everything is 
assembler code, be it handwritten or GCC-generated.  Have a look at how 
GCC sets flags for your section, by running `gcc -S' and examining the 
output.

  Maciej

Re: [RFC] Add __initbss section

[Ralf Baechle]

On Sun, Oct 14, 2007 at 09:42:08PM +0200, Franck Bui-Huu wrote:

> > .exit.data and .exit.text may reference each other.  __exit functions
> > generally get compiled into .exit.text but some constructs such as jump
> > tables for switch() constructs may be compiled into address tables which
> > gcc unfortunately will put into .rodata, so .rodata will end up
> > referencing function addresses in .exit.text which makes ld unhappy if
> > .exit.text was discarded.  So until this is fixed in gcc we can't
> > discard exit code, unfortunately.
> > 
> 
> Thanks for the details.
> 
> I actually don't see any point to move these tables in .rodata since
> they're part of the code...

As I recall the argumentation was they should go there because that section
can be marked no-exec.  Which isn't terribly useful on MIPS where only
very few processors have the no-exec capability.

Anyway, I guess it takes somebody to cook a patch :-)

  Ralf

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Maciej W. Rozycki wrote:
> On Sun, 14 Oct 2007, Franck Bui-Huu wrote:
> 
>>>  I guess for a bss-type section you want to use something like:
>>>
>>> 	.section .init.bss,"aw",@nobits
>>>
>> Sorry but I'm missing your point here. This indeed should be added
>> for assembler code but I don't see how it's related with the kernel
>> image size difference I was seeing.
> 
>  Well, otherwise the section is marked as containing data and therefore 
> taking space in the image.

Well, since .init.bss is declared as follow:

	.init.bss (NOLOAD) : {
		...
	}

data should not take any space in the image...

> 
>  What do you mean by "assembler code" in this context, BTW?

I meant to be able to put data into .init.bss section from assembly
code (*.S files) like __INITDATA does for .init.data section.

		Franck

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Ralf Baechle wrote:
> As I recall the argumentation was they should go there because that section
> can be marked no-exec.

Are these switch tables really different from ELF .plt or .got ?

		Franck

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Mon, 15 Oct 2007, Franck Bui-Huu wrote:

> Well, since .init.bss is declared as follow:
> 
> 	.init.bss (NOLOAD) : {
> 		...
> 	}
> 
> data should not take any space in the image...

 The above only marks it as unloadable (cf. e.g. debugging information).  
It is still there.

> I meant to be able to put data into .init.bss section from assembly
> code (*.S files) like __INITDATA does for .init.data section.

 That does not differ from what has to be done for any other language -- 
ultimately `gas', which is responsible for such arrangements, has to see 
an appropriate ".section" directive.

  Maciej

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Maciej W. Rozycki wrote:
> On Mon, 15 Oct 2007, Franck Bui-Huu wrote:
> 
>> Well, since .init.bss is declared as follow:
>>
>> 	.init.bss (NOLOAD) : {
>> 		...
>> 	}
>>
>> data should not take any space in the image...
> 
>  The above only marks it as unloadable (cf. e.g. debugging information).  
> It is still there.
> 

So it seems there is no way from a linker script to specify that a
section has the nobits type, is it ?

After spending some fun time trying several different configurations
with gcc and ld, I noticed that gcc makes a section with @nobits
attribute if the section name starts with .bss.*

So calling .bss.init instead of .init.bss makes gcc do the right
thing. Here is a bit of tlbex.s:

		.word	0
		.section	.bss.init,"aw",@nobits
		.align	2
		.type	tlb_handler, @object
		.size	tlb_handler, 512
	tlb_handler:
		.space	512
		.align	2
		.type	labels, @object
		.size	labels, 1024
	labels:
		.space	1024


Another test I did is to put .init.bss (not .bss.init) section right
before .bss section in order to have only one segment to load. And it
makes magically ld do the right thing. I must admit that I don't
understand why, and the lack of documentation doesn't help...

Unfortunately I don't know if we can rely on one of these
behaviours. IOW if they're going to work with all supported versions
of gcc/ld.

Anyway, I'll resubmit a new patchset for futher discussion.

		Franck

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Thu, 18 Oct 2007, Franck Bui-Huu wrote:

> So it seems there is no way from a linker script to specify that a
> section has the nobits type, is it ?

 Well, you cannot switch section types in a linker script at all in 
general.  The place to do it is always in the assembler, which is the tool 
which actually emits this data.

> After spending some fun time trying several different configurations
> with gcc and ld, I noticed that gcc makes a section with @nobits
> attribute if the section name starts with .bss.*

 Exactly how GCC sets section flags is mostly determined by 
default_section_type_flags() in gcc/varasm.c; some flags are set elsewhere 
too.  This is with HEAD of GCC.

> Another test I did is to put .init.bss (not .bss.init) section right
> before .bss section in order to have only one segment to load. And it
> makes magically ld do the right thing. I must admit that I don't
> understand why, and the lack of documentation doesn't help...

 Hmm, isn't what `info ld' says enough?  I think a look at what the SysV 
ABI documents say about ELF could clarify some matters too.  Also have a 
look at how sections are mapped to segments as after the final link it is 
segments that matter -- `readelf -l' will do.

> Unfortunately I don't know if we can rely on one of these
> behaviours. IOW if they're going to work with all supported versions
> of gcc/ld.

 Probably -- I guess you only need to check what GCC 3.2 did.  Or you 
could use a hack with __attribute__((section())) like one I did for 
merging strings -- unlike with SECTION_BSS I think there is no way to 
request (SECTION_MERGE | SECTION_STRINGS) in an implicit way anyhow.

  Maciej

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Maciej W. Rozycki wrote:
> On Thu, 18 Oct 2007, Franck Bui-Huu wrote:
>> After spending some fun time trying several different configurations
>> with gcc and ld, I noticed that gcc makes a section with @nobits
>> attribute if the section name starts with .bss.*
> 
>  Exactly how GCC sets section flags is mostly determined by 
> default_section_type_flags() in gcc/varasm.c; some flags are set elsewhere 
> too.  This is with HEAD of GCC.
> 

It seems that we can rely on this behaviour. I looked at gcc 4.1.2/3.2
sources and they made a section part of .bss if the section name starts
with ".bss.".

>> Another test I did is to put .init.bss (not .bss.init) section right
>> before .bss section in order to have only one segment to load. And it
>> makes magically ld do the right thing. I must admit that I don't
>> understand why, and the lack of documentation doesn't help...
> 
>  Hmm, isn't what `info ld' says enough?

Hmm, I'm must be blind but I missed that each time I read it. Could
you point out the section number please ?

OK, I think the best thing to do now is to respin the patchset and
submit it linux arch mailing list.

thanks,

		Franck

Re: [RFC] Add __initbss section

[Maciej W. Rozycki]

On Sun, 4 Nov 2007, Franck Bui-Huu wrote:

> >  Hmm, isn't what `info ld' says enough?
> 
> Hmm, I'm must be blind but I missed that each time I read it. Could
> you point out the section number please ?

 That's section 3.8, I would guess, but if what you are looking for is not 
there, then perhaps the description could be improved.

  Maciej

Re: [RFC] Add __initbss section

[Franck Bui-Huu]

Maciej W. Rozycki wrote:
> On Sun, 4 Nov 2007, Franck Bui-Huu wrote:
> 
>>>  Hmm, isn't what `info ld' says enough?
>> Hmm, I'm must be blind but I missed that each time I read it. Could
>> you point out the section number please ?
> 
>  That's section 3.8, I would guess, but if what you are looking for is not 
> there, then perhaps the description could be improved.
> 

After reading section 3.8 "PHDRS Command", I still fail to find
something interesting.

		Franck