Is arch/x86/boot/main.c main obsolete?

Is arch/x86/boot/main.c main obsolete?

[ebashinskii]

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CPU: Intel x86 KabyLake. I'm using 5.17 Kernel built from upstream and GRUB 2.04

When debugging the kernel initialization process I found out that there is a function void main(void) in the source file arch/x86/boot/main.c (source link: https://elixir.bootlin.com/linux/v5.17/source/arch/x86/boot/main.c#L134) which seems to be the entry point which the boot loader is supposed to jump into.

But objdump-ing the Kernel image shows that there is no such symbol as main. GDB attached to QEMU does not know about this symbol either.

The earliest stage of Kernel Initialization I could manage to catch with GDB is x86_64_start_kernel (source link: https://elixir.bootlin.com/linux/v5.17/source/arch/x86/kernel/head64.c#L467)

The question is what function GRUB actually jump into passing control to the Kernel code and what is the purpose of arch/x86/boot/main.c::main?

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Re: Is arch/x86/boot/main.c main obsolete?

[Aruna Hewapathirane]

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On Sat, Jun 25, 2022 at 7:35 PM ebashinskii <ebashinskii@proton.me> wrote:

> CPU: Intel x86 KabyLake. I'm using 5.17 Kernel built from upstream and
> GRUB 2.04
>
> When debugging the kernel initialization process I found out that there is
> a function *void main(void)* in the source file *arch/x86/boot/main.c* (source
> link:
> https://elixir.bootlin.com/linux/v5.17/source/arch/x86/boot/main.c#L134) which
> seems to be the entry point which the boot loader is supposed to jump into.
>
> But *objdump*-ing the Kernel image shows that there is no such symbol as
> *main*. GDB attached to QEMU does not know about this symbol either.
>
> The earliest stage of Kernel Initialization I could manage to catch with
> GDB is *x86_64_start_kernel* (source link:
> https://elixir.bootlin.com/linux/v5.17/source/arch/x86/kernel/head64.c#L467
> )
>
> The question is what function GRUB actually jump into passing control to
> the Kernel code and what is the purpose of *arch/x86/boot/main.c::main*?
>

This should answer most of the questions you may have about the kernel boot
process:
https://0xax.gitbooks.io/linux-insides/content/Booting/

:-)



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Re: Is arch/x86/boot/main.c main obsolete?

[ebashinskii]

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I tried to set up a breakpoint inGDB and to lookup void main(void) and _start with objdumpon the kernel image without any success.

Actually it clearly says that the control Jump to the C code inarch/x86/boot/main.c. But as I mentioned before there's simply no such symbol as void main(void) in the Kernel image.

How can it jump to something that does not exist?

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Re: Is arch/x86/boot/main.c main obsolete?

[jim.cromie]

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On Sat, Jun 25, 2022 at 5:35 PM ebashinskii <ebashinskii@proton.me> wrote:

> CPU: Intel x86 KabyLake. I'm using 5.17 Kernel built from upstream and
> GRUB 2.04
>
> When debugging the kernel initialization process I found out that there is
> a function *void main(void)* in the source file *arch/x86/boot/main.c* (source
> link:
> https://elixir.bootlin.com/linux/v5.17/source/arch/x86/boot/main.c#L134) which
> seems to be the entry point which the boot loader is supposed to jump into.
>
> But *objdump*-ing the Kernel image shows that there is no such symbol as
> *main*. GDB attached to QEMU does not know about this symbol either.
>
> The earliest stage of Kernel Initialization I could manage to catch with
> GDB is *x86_64_start_kernel* (source link:
> https://elixir.bootlin.com/linux/v5.17/source/arch/x86/kernel/head64.c#L467
> )
>
>

What are you trying to do that start-kernel is insufficiently early ?

main.c 's that you dont hit dont really matter do they ?

fwiw, I really like virtme

    virtme-run $KRUN_SHOW $KRUN_STDS $KDBG_OPTS $* $QM_OPTS --smp 3 -s -S

the -s -S stops me early enough to anything Ive needed to so far.

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