Re : Re : Re : [PATCH] Compressed ia32 ELF file generation for loading by Gujin 1/3

Re : Re : Re : [PATCH] Compressed ia32 ELF file generation for loading by Gujin 1/3

[Etienne Lorrain]

Eric W. Biederman wrote:
> Etienne Lorrain <etienne_lorrain@yahoo.fr> writes:
>>  Well, if the function called at offset 0 in the real-mode section return
>>  non zero, that is probably an error - that is Gujin interface, but do not ask
>>  me to change other bootloaders to handle that.
>>  This function is called with few parameters, one is a string pointer
>>  if the function do not want to print the problem report itself.
>>  The function is called with an offset of zero in its own intel
>>  segment.
>
> So I think there are some backwards compatibility issues with your
> real mode code interface.  Setting a new flag that says to return instead
> of printing an error message and halting would be one way to handle
> this.

 I am not sure to understand: Gujin calls a function inside the ELF real
mode program header of the Linux kernel. All the system is currently
in real mode. There isn't any limitation in this function of the kernel to
decide to print some message and not return at all, this function can
selectively do so by reading which bootloader ID it is using (another
parameter). It is not done mostly because "printf" is difficult to do
in assembler, Gujin has no problem.

> Is your real mode C code section relative such that it can be loaded
> at different addresses and still work?

  The code is relative - but not the data (first 4 Kbytes at %ds:0 and stack
 available) - but the whole lot at any segment boundary, i.e. every 16 bytes.
 Else Gujin would not work under DOS.

> The program header is for executable loaders the section header is for
> linkers and the section header is optional in PT_LOAD and PT_DYN
> executables.  Use of the section header by a loader is a bug.

  Unless if there is a problem, Gujin uses only the program header;
 it has a look in the section header just in case - that can be removed
 easily. I was wondering about a relocatable image there - but I do
 not know enough for that.

> There have been limitations but mostly with respect to page table size
> and the like but they were not limitations a bootloader had to deal with.

Was talking about this comment, but it is old:
/* 0x28000*16 = 2.5 MB, conservative estimate for the current maximum */
http://lxr.linux.no/source/arch/i386/boot/tools/build.c?v=2.4.28

> >   Well, you can generate with the proposed patch and boot with SYSLINUX,
> >  Grub and LILO, but trying to clean Linux real-mode code is where I begun.
>
> I haven't looked yet.  But I believe this is something that we can do,
> so long as cleaning and feature enhancement are not mixed in a bad way.
> Phrasing this another way.  What we can do with the interface is
> determined our interface to existing bootloaders and what bootloaders
> exist.  Basically it is the rule that we need to preserve backwards
> compatibility and changes generally need to be evolutionary ones.

  Well, if you want to preserve compatibility with other bootloaders,
 it is probably possible to put some source around the ELF kernel file,
 mostly taken from Gujin if you want (GPL), but I wonder why you would
 like to be compatible with LILO.
 Modifying the linux real mode assembler, nobody could even want to.

 Etienne.






	

	
		
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Re: Re : Re : Re : [PATCH] Compressed ia32 ELF file generation for loading by Gujin 1/3

[Eric W. Biederman]

Etienne Lorrain <etienne_lorrain@yahoo.fr> writes:

>  I am not sure to understand: Gujin calls a function inside the ELF real
> mode program header of the Linux kernel. All the system is currently
> in real mode. There isn't any limitation in this function of the kernel to
> decide to print some message and not return at all, this function can
> selectively do so by reading which bootloader ID it is using (another
> parameter). It is not done mostly because "printf" is difficult to do
> in assembler, Gujin has no problem.

This code is currently completely Gujin specific.  If concieved as a
replacement for setup.S it has a chance of passing review.

>> Is your real mode C code section relative such that it can be loaded
>> at different addresses and still work?
>
>   The code is relative - but not the data (first 4 Kbytes at %ds:0 and stack
>  available) - but the whole lot at any segment boundary, i.e. every 16 bytes.
>  Else Gujin would not work under DOS.

Ok.  Similar restrictions to the current real mode code.

>> The program header is for executable loaders the section header is for
>> linkers and the section header is optional in PT_LOAD and PT_DYN
>> executables.  Use of the section header by a loader is a bug.
>
>   Unless if there is a problem, Gujin uses only the program header;
>  it has a look in the section header just in case - that can be removed
>  easily. I was wondering about a relocatable image there - but I do
>  not know enough for that.

When you are relocatable no relocations are processed and all addresses in
the program header are slid by some fixed amount (preserving alignment)
after that the relocated object has to work out the relocations by itself.

>> >   Well, you can generate with the proposed patch and boot with SYSLINUX,
>> >  Grub and LILO, but trying to clean Linux real-mode code is where I begun.
>>
>> I haven't looked yet.  But I believe this is something that we can do,
>> so long as cleaning and feature enhancement are not mixed in a bad way.
>> Phrasing this another way.  What we can do with the interface is
>> determined our interface to existing bootloaders and what bootloaders
>> exist.  Basically it is the rule that we need to preserve backwards
>> compatibility and changes generally need to be evolutionary ones.
>
>   Well, if you want to preserve compatibility with other bootloaders,
>  it is probably possible to put some source around the ELF kernel file,
>  mostly taken from Gujin if you want (GPL), but I wonder why you would
>  like to be compatible with LILO.

LILO is a lot saner then Grub, and it still supports more filesystems...
Just because it memorizes it all before you shut down the system for
simplicity doesn't mean lilo is worse.

>  Modifying the linux real mode assembler, nobody could even want to.

I have several times.  It's just code.  But I do agree it will likely
be more maintainable if we can convert it to C.

On that same token I wrote a compiler in romcc in another context so I
didn't have to write so much assembly.  And for maintainability and
comprehensibility it was a big help.

Eric

Re: Re : Re : Re : [PATCH] Compressed ia32 ELF file generation for loading by Gujin 1/3

[H. Peter Anvin]

Eric W. Biederman wrote:
> 
>>  Modifying the linux real mode assembler, nobody could even want to.
> 
> I have several times.  It's just code.  But I do agree it will likely
> be more maintainable if we can convert it to C.
> 
> On that same token I wrote a compiler in romcc in another context so I
> didn't have to write so much assembly.  And for maintainability and
> comprehensibility it was a big help.
> 

I've long wished that someone would do a proper 16-bit x86 port of gcc; 
however, the .code16gcc is usually good enough, although it produces 
code which is a lot bigger than it needs to be.

	-hpa

Re: RE : Re: Re : [PATCH] Compressed ia32 ELF file generation for loading by Gujin 1/3

[Eric W. Biederman]

Etienne Lorrain <etienne_lorrain@yahoo.fr> writes:

> Eric W. Biederman wrote:
>> Etienne Lorrain writes:
>> >   Well, a self relocating image cannot be an ELF file because the code
>> >  to relocate the ELF cannot be executed at the wrong place.
>> >  If relocation is needed, I would better like not to link vmlinux at a
>> >  fixed address first. In fact I wonder if we are talking of the same
>> >  kind of relocation: you seem to talk about "ld --pic-executable" while
>> >  I am thinking of "ld -r" to "locate" it at the bootloader loading time.
>> >   The main problem I see is that I do not have the code for that, and
>> >  I am going deeper/earlier into the generation of vmlinux, while comments
>> >  are "already you are too early, loading an ELF file is too complex for
>> >  a bootloader". The solution I have already is working.
>> 
>> Being very clear.  ld --pic-executable or ld -shared is essentially
>> what we are talking when we are discussing building a relocatable
>> kernel.  Something with the properties of an ELF ET_DYN executable
>> that does not use an interpreter.  ld.so is the only common executable
>> of this type in linux.
>
>  So we are talking of two different things.
>  You want to _compile_ the totality of the kernel as code and data
> relocatable - right now only modules are compiled that way.
>  On IA32 architecture, there is no problem having the code relocatable,
> because most/all calls and jumps are PC relative, so very few instructions
> need to be different.
>  The problem is having the data relocatable, then every access to memory
> has to be relative to a register, and so the register EBX is reserved
> for that by the compiler.
>  The IA32 processor does not have enough registers already, removing one
> of them makes the assembly seriously worse - mostly when dealing with
> 64 bits "long long" - you can only get two of them in register and a
> simple addition produces long and slow code.
>  Having the data relocatable could be done by using the base address of
> IA32 segments, but this does not seem to be the preferred method because
> segments use seems to slow down the processor a lot more.

There are two more options in addition to segments:
- Running at a fixed virtual address.
- Processing relocation entries to modify the initial code/data to
  live where we want.  We don't have the that shared libraries do of wanting
  to share text/data pages loaded at different addresses.

>  What I was talking of was "loading" a non relocatable kernel (compiled
> as usual without -fPIC/-shared) at any address, so not inducing slow
> down whatever the base address used. I said that it is possible, maybe
> quite easy to do so, but I do not have the code for it - and people
> say right now that an ELF bootloader is not what they want to.

The thing to pick up on is that we really don't want the gcc flag
-fPIC but if ld wasn't an idiot we would want the linker flag -shared.
Anyway we have working code merged that processes all of the
relocation entries and modifies the kernel to run properly where we
load it at.  This has all been done and merged at this point.  It's
only available to a bzImage loader but it is all there.  Currently the
only overhead is something like 5 or 10 percent overhead to hold the
relocation entries.

The only thing currently compiled with -fPIC is the kernel gzip
decompressor and it is carefully coded so it doesn't generate any
relocation entries that need processing.  It is just throw away code
so we don't much care about it's efficiency.

>  Now, if people wants to switch to a completely relocatable kernel,
> because they do not mind the slow down (that I am not really able
> to quantify) - and because anyway most of their kernel is modules,
> then Gujin can relocate that without _any_ problem; in fact it
> does already load in intermediate (HIMEM malloc'ed) memory under
> DOS to relocate it later (called 'late relocation' in Gujin code,
> done every times the code is not at the right address in memory,
> interrupt disabled and in protected mode).

Yes.  That is roughly what we are saying.  We want that at least
as a well defined option.

>  Loading and executing at whatever address is not a problem,
> and should be doable already in the real-mode function of the
> kernel called by Gujin, by modifying few elements of the
> structure passed in parameter of this real-mode function:
> it should be done after loading the E820 memory map for the
> BIOS parameter page.
> I am reading right now Gujin source and note that you may want
> to access array elf_reloc[] and variable nb_elf_reloc in this
> Linux function - will be added in next Gujin version.
>  You already can access type LOADER_str parameter "loader"
> where you can get runadr which can overwrite the ELF entry
> point address. The old ELF header is at loader.load_address
> if it is needed.
>
>  The main problem Gujin has, is to decide where to load a
> relocatable kernel, so this policy is left to the kernel and
> not to the user. For instance and as you made me think of, if you
> do not want for security reasons that the kernel memory could
> be modified by an attacker by using the legacy DMA, then you
> want to load it at an address which cannot be modified this way,
> i.e. over 16 Mbytes. Note that some chipset had an extension I/O
> address to extend this legacy DMA to 32 bits addresses, but it
> has not been documented and probably not implemented widely.
>
>  I do not know how many people are like me, but I am currently
> typing on a monolitic kernel because I find it a _lot_ simpler
> to do a ".config" once and make it slowly evolve with new kernel
> versions (answering No to most of the questions), than managing
> hundreds of module files each time I regenerate/remove a kernel.
>  The kernel I am working on is not relocatable because - well
> I do not have crash to debug...

Yes.   In this sense I'm a tool builder.  I build general capabilities
that can be used to solve specific problems.  A relocatable kernel
is a general capability and the only interesting problem to date
has been taking crash dumps.  To reduce the maintenance overhead I
try and remove the cases where you need an extra config option for
the new functionality.  If there is not overhead a config option just
makes things harder to test.

>  So I think that from the Gujin point of view, everything you need
> is done and working (but the access to elf_reloc)- please explain what
> you want more if not. If you want a solution to load now, you can
> use patch linux-2.6.20-rc5 in sourceforge, which uses an intermediate
> binary dump of the ELF file, and can be relocated the non ELF way.

This requires a more detailed examination.  The previous conversation
indicated that if we could replace setup.S, video.S and edd.S with
C versions it would be very interesting.  If we happen to change the
link so they are available in an ELF executable so much the better.

If we wind up adding two versions of the real mode code one written
in assembly and one written in C accessible by different means
by different bootloaders that is not interesting.

>  So, why do I want an ELF file loaded by a bootloader?

>  The main reason is to be able to use all the usual ELF tools to do
> all sort of things, like showing segments size and base address with
> "objdump" and "readelf", adding and extracting sections with "strip"
> and "objcopy", and dumping section content with either "objdump -m i8086"
> or "objdump" alone to display the assembly instructions after link,
> for real-mode or protected-mode, with symbols.
>  For all that, I need a 100% compatible ELF file structure - that is
> what I have already.

Yes.  That is nice. 

>  I know that to load this ELF kernel file the user will need a different
> loader like Gujin or your kexec. I know that I am asking you not to treat
> an impossible action (loading a section at address 0 over the interrupt
> table), and I say please.

It is my belief that we can solve this without hacks, and still have
a useable structure.

[snip]
>  So technically - and if I understand you correctly (feel free to correct
>  me) - you ask me to put the real mode code into the NOTE section, by
>  adding a few words header and concatening the code.
>
>  There is two way to do so, the first being simply to replace in
> vmlinux.lds.S (in the patch I sent) the lines:
>   .realmode 0 : AT (ADDR(.bss) + SIZEOF(.bss) - LOAD_OFFSET) {
>   ..... *(.init.text16) ...
>   } :text16 = 0x9090
> by something like:
>   .realmode 0 : AT (ADDR(.bss) + SIZEOF(.bss) - LOAD_OFFSET) {
>   LONG(8)
>   LONG(SIZEOF (.realmode))
>   LONG(1)
>   BYTE('R') BYTE('E') BYTE('A') BYTE('L')
>   BYTE('M') BYTE('O') BYTE('D') BYTE('E')
>   ..... *(.init.text16) ...
>   } :note = 0x9090

That would certainly get the job done.

>  The other way to do is to produce an independant link for the real-mode
> program (linked at base address 0), convert it to binary and include
> the resulting block into the NOTE segment. First that is a lot more
> complex (it can be done but is more complex), and two I loose all
> the debuging information I wanted in the ELF file.
>  I can still see the real-mode assembly by forcing objdump to interpret
> the NOTE section as real-mode code, but the offset calculated for
> assembly calls and jumps is wrong, I lost all symbols, in short it
> goes against what I wanted initially.

So I really don't care if we move whole real mode section into a note
or if we just put a pointer to it into a note.  What is important is
that we use a note to find it.

Which means that we could do something goofy in the linker script
like we do with the current vdso.  So we could give it a virtual
address of 0 and a physical address in the init code section.
Allowing us to load it normally and then just drop it later.  Not
perfect but something that would not need special treatment by
a bootloader unless it wanted to copy the data from there into
some location in low memory and execute it.

For me the objective is not so much reusing the existing tools
(although that is a plus) but more to be able to build a unified
binary that can be used for everything, and will give us the freedom
to do interesting things with the kernel in the future, and hopefully
something that is more or less usable by portable bootloaders.  Having
a different file format and different rules for different
architectures is a pain.

Eric

RE : Re: Re : [PATCH] Compressed ia32 ELF file generation for loading by Gujin 1/3

[Etienne Lorrain]

Eric W. Biederman wrote:
> Etienne Lorrain writes:
> >   Well, a self relocating image cannot be an ELF file because the code
> >  to relocate the ELF cannot be executed at the wrong place.
> >  If relocation is needed, I would better like not to link vmlinux at a
> >  fixed address first. In fact I wonder if we are talking of the same
> >  kind of relocation: you seem to talk about "ld --pic-executable" while
> >  I am thinking of "ld -r" to "locate" it at the bootloader loading time.
> >   The main problem I see is that I do not have the code for that, and
> >  I am going deeper/earlier into the generation of vmlinux, while comments
> >  are "already you are too early, loading an ELF file is too complex for
> >  a bootloader". The solution I have already is working.
> 
> Being very clear.  ld --pic-executable or ld -shared is essentially
> what we are talking when we are discussing building a relocatable
> kernel.  Something with the properties of an ELF ET_DYN executable
> that does not use an interpreter.  ld.so is the only common executable
> of this type in linux.

 So we are talking of two different things.
 You want to _compile_ the totality of the kernel as code and data
relocatable - right now only modules are compiled that way.
 On IA32 architecture, there is no problem having the code relocatable,
because most/all calls and jumps are PC relative, so very few instructions
need to be different.
 The problem is having the data relocatable, then every access to memory
has to be relative to a register, and so the register EBX is reserved
for that by the compiler.
 The IA32 processor does not have enough registers already, removing one
of them makes the assembly seriously worse - mostly when dealing with
64 bits "long long" - you can only get two of them in register and a
simple addition produces long and slow code.
 Having the data relocatable could be done by using the base address of
IA32 segments, but this does not seem to be the preferred method because
segments use seems to slow down the processor a lot more.

 What I was talking of was "loading" a non relocatable kernel (compiled
as usual without -fPIC/-shared) at any address, so not inducing slow
down whatever the base address used. I said that it is possible, maybe
quite easy to do so, but I do not have the code for it - and people
say right now that an ELF bootloader is not what they want to.

 Now, if people wants to switch to a completely relocatable kernel,
because they do not mind the slow down (that I am not really able
to quantify) - and because anyway most of their kernel is modules,
then Gujin can relocate that without _any_ problem; in fact it
does already load in intermediate (HIMEM malloc'ed) memory under
DOS to relocate it later (called 'late relocation' in Gujin code,
done every times the code is not at the right address in memory,
interrupt disabled and in protected mode).
 Loading and executing at whatever address is not a problem,
and should be doable already in the real-mode function of the
kernel called by Gujin, by modifying few elements of the
structure passed in parameter of this real-mode function:
it should be done after loading the E820 memory map for the
BIOS parameter page.
I am reading right now Gujin source and note that you may want
to access array elf_reloc[] and variable nb_elf_reloc in this
Linux function - will be added in next Gujin version.
 You already can access type LOADER_str parameter "loader"
where you can get runadr which can overwrite the ELF entry
point address. The old ELF header is at loader.load_address
if it is needed.

 The main problem Gujin has, is to decide where to load a
relocatable kernel, so this policy is left to the kernel and
not to the user. For instance and as you made me think of, if you
do not want for security reasons that the kernel memory could
be modified by an attacker by using the legacy DMA, then you
want to load it at an address which cannot be modified this way,
i.e. over 16 Mbytes. Note that some chipset had an extension I/O
address to extend this legacy DMA to 32 bits addresses, but it
has not been documented and probably not implemented widely.

 I do not know how many people are like me, but I am currently
typing on a monolitic kernel because I find it a _lot_ simpler
to do a ".config" once and make it slowly evolve with new kernel
versions (answering No to most of the questions), than managing
hundreds of module files each time I regenerate/remove a kernel.
 The kernel I am working on is not relocatable because - well
I do not have crash to debug...

 So I think that from the Gujin point of view, everything you need
is done and working (but the access to elf_reloc)- please explain what
you want more if not. If you want a solution to load now, you can
use patch linux-2.6.20-rc5 in sourceforge, which uses an intermediate
binary dump of the ELF file, and can be relocated the non ELF way.

> >  In fact, thinking more about that, I am going back to my implementation
> >  of it, because on ia32 the interrupt vectors are at address zero and it is
> >  obviouly an invalid address to load an ELF for this architecture.
> 
> No special games no special rules with the well defined ELF components
> either add a note that you can define all of the semantics yourself
> or don't do it.  That is what the notes are there for.

 So, why do I want an ELF file loaded by a bootloader?
 The main reason is to be able to use all the usual ELF tools to do
all sort of things, like showing segments size and base address with
"objdump" and "readelf", adding and extracting sections with "strip"
and "objcopy", and dumping section content with either "objdump -m i8086"
or "objdump" alone to display the assembly instructions after link,
for real-mode or protected-mode, with symbols.
 For all that, I need a 100% compatible ELF file structure - that is
what I have already.
 I do _not_ need to use the Linux loader at the prompt of a Xterm for
this file, if I try I have this very informative error message:
[etienne@localhost linux-2.6.20]$ ./vmlinux.elf 
Killed
[etienne@localhost linux-2.6.20]$ 
 I know that to load this ELF kernel file the user will need a different
loader like Gujin or your kexec. I know that I am asking you not to treat
an impossible action (loading a section at address 0 over the interrupt
table), and I say please.

> >  But for the linker, it is the right address to link it (being an offset
> >  into a non-null segment in real mode), and because the entry point has
> >  to be zero (I cannot use the ELF entry value) the program header base
> >  address has to be zero.
> 
> Agreed. When the object file is linked using offset 0, and letting the
> real mode segments do have different bases to do your relocation is fine.

 Nice to agree.

> We have been very sparse on the usage of ELF notes but yes they exist
> and yes people do look at them.  Please dig up a copy of the ELF spec
> and read up on them or look at etherboot for an example.

 I've read it again from cover to cover, version 1.2, pretty boring
 stuff, isn't it?
 So technically - and if I understand you correctly (feel free to correct
 me) - you ask me to put the real mode code into the NOTE section, by
 adding a few words header and concatening the code.

 There is two way to do so, the first being simply to replace in
vmlinux.lds.S (in the patch I sent) the lines:
  .realmode 0 : AT (ADDR(.bss) + SIZEOF(.bss) - LOAD_OFFSET) {
  ..... *(.init.text16) ...
  } :text16 = 0x9090
by something like:
  .realmode 0 : AT (ADDR(.bss) + SIZEOF(.bss) - LOAD_OFFSET) {
  LONG(8)
  LONG(SIZEOF (.realmode))
  LONG(1)
  BYTE('R') BYTE('E') BYTE('A') BYTE('L')
  BYTE('M') BYTE('O') BYTE('D') BYTE('E')
  ..... *(.init.text16) ...
  } :note = 0x9090
That has the main advantage to keep producing a single link, but the
major disadvantage that this code is no more linked at 0 but offseted
by the header of 5 longs, the call to the function has to be offseted,
and it may not work nicely when another developper comes in few years
and add another few Kbytes note - everything will work if his note
is added after mine but break badly if it is added before mine.
 The order it is processed is under the linker control, and anybody
could add a note in C by declaring a structure in the .note section.
 You will probably agree with me that it is not an acceptable solution.

 The other way to do is to produce an independant link for the real-mode
program (linked at base address 0), convert it to binary and include
the resulting block into the NOTE segment. First that is a lot more
complex (it can be done but is more complex), and two I loose all
the debuging information I wanted in the ELF file.
 I can still see the real-mode assembly by forcing objdump to interpret
the NOTE section as real-mode code, but the offset calculated for
assembly calls and jumps is wrong, I lost all symbols, in short it
goes against what I wanted initially.

  Thanks,
  Etienne.


	

	
		
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