Re: [PATCH v9 06/11] arm64: kexec_file: allow for loading Image-format kernel

[AKASHI Takahiro]

James,

On Fri, May 11, 2018 at 06:07:06PM +0100, James Morse wrote:
> Hi Akashi,
> 
> On 07/05/18 08:21, AKASHI Takahiro wrote:
> > On Tue, May 01, 2018 at 06:46:11PM +0100, James Morse wrote:
> >> On 25/04/18 07:26, AKASHI Takahiro wrote:
> >>> This patch provides kexec_file_ops for "Image"-format kernel. In this
> >>> implementation, a binary is always loaded with a fixed offset identified
> >>> in text_offset field of its header.
> 
> >>> diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
> >>> index e4de1223715f..3cba4161818a 100644
> >>> --- a/arch/arm64/include/asm/kexec.h
> >>> +++ b/arch/arm64/include/asm/kexec.h
> >>> @@ -102,6 +102,56 @@ struct kimage_arch {
> >>>  	void *dtb_buf;
> >>>  };
> >>>  
> >>> +/**
> >>> + * struct arm64_image_header - arm64 kernel image header
> >>> + *
> >>> + * @pe_sig: Optional PE format 'MZ' signature

To be precise, this is NOT a PE signature but MS-DOS header's magic.
(There is another "PE" signature in PE COFF file header pointed to by
'pe_header'.)
I will correct its name.

> >>> + * @branch_code: Instruction to branch to stext
> >>> + * @text_offset: Image load offset, little endian
> >>> + * @image_size: Effective image size, little endian
> >>> + * @flags:
> >>> + *	Bit 0: Kernel endianness. 0=little endian, 1=big endian
> >>
> >> Page size? What about 'phys_base'?, (whatever that is...)
> >> Probably best to refer to Documentation/arm64/booting.txt here, its the
> >> authoritative source of what these fields mean.
> > 
> > While we don't care other bit fields for now, I will add the reference
> > to the Documentation file.
> 
> Thanks, I don't want to create a second, incomplete set of documentation!

I will leave a minimum of description of parameters here.

> 
> 
> >>> +	u64 reserved[3];
> >>> +	u8 magic[4];
> >>> +	u32 pe_header;
> >>> +};
> >>
> >> I'm surprised we don't have a definition for this already, I guess its always
> >> done in asm. We have kernel/image.h that holds some of this stuff, if we are
> >> going to validate the flags, is it worth adding the code there, (and moving it
> >> to include/asm)?
> > 
> > A comment at the beginning of this file says,
> >     #ifndef LINKER_SCRIPT
> >     #error This file should only be included in vmlinux.lds.S
> >     #endif
> > Let me think about.
> 
> Ah, I missed that.
> 
> Having two definitions of something makes me nervous that they can become
> different... looks like that header belongs to the linker, and shouldn't be used
> here then.

OK.

> 
> >> I guess you skip the MZ prefix as its not present for !EFI?

Correct, but MZ checking in probe function is just an informative message.

> > 
> > CONFIG_KEXEC_IMAGE_VERIFY_SIG depends on the fact that the file
> > format is PE (that is, EFI is enabled).
> 
> So if the signature checking is enabled, its already been checked.

The signature, either MZ or PE, in a file will be actually checked
in verify_pefile_signature().

> 
> >> Could we check branch_code is non-zero, and text-offset points within image-size?
> > 
> > We could do it, but I don't think this check is very useful.
> > 
> >>
> >> We could check that this platform supports the page-size/endian config that this
> >> Image was built with... We get a message from the EFI stub if the page-size
> >> can't be supported, it would be nice to do the same here (as we can).
> > 
> > There is no restriction on page-size or endianness for kexec.
> 
> No, but it won't boot if the hardware doesn't support it. The kernel will spin
> at a magic address that is, difficult, to debug without JTAG. The bug report
> will be "it didn't boot".

OK.
Added sanity checks for cpu features, endianness as well as page size.

> 
> > What will be the purpose of this check?
> 
> These values are in the header so that the bootloader can check them, then print
> a meaningful error. Here, kexec_file_load() is playing the part of the bootloader.
> 
> I'm assuming kexec_file_load() can only be used to kexec linux... unlike regular
> kexec. Is this where I'm going wrong?
> 
> 
> >>> diff --git a/arch/arm64/kernel/kexec_image.c b/arch/arm64/kernel/kexec_image.c
> >>> new file mode 100644
> >>> index 000000000000..4dd524ad6611
> >>> --- /dev/null
> >>> +++ b/arch/arm64/kernel/kexec_image.c
> >>> @@ -0,0 +1,79 @@
> >>
> >>> +static void *image_load(struct kimage *image,
> >>> +				char *kernel, unsigned long kernel_len,
> >>> +				char *initrd, unsigned long initrd_len,
> >>> +				char *cmdline, unsigned long cmdline_len)
> >>> +{
> >>> +	struct kexec_buf kbuf;
> >>> +	struct arm64_image_header *h = (struct arm64_image_header *)kernel;
> >>> +	unsigned long text_offset;
> >>> +	int ret;
> >>> +
> >>> +	/* Load the kernel */
> >>> +	kbuf.image = image;
> >>> +	kbuf.buf_min = 0;
> >>> +	kbuf.buf_max = ULONG_MAX;
> >>> +	kbuf.top_down = false;
> >>> +
> >>> +	kbuf.buffer = kernel;
> >>> +	kbuf.bufsz = kernel_len;
> >>> +	kbuf.memsz = le64_to_cpu(h->image_size);
> >>> +	text_offset = le64_to_cpu(h->text_offset);
> >>> +	kbuf.buf_align = SZ_2M;
> >>
> >>> +	/* Adjust kernel segment with TEXT_OFFSET */
> >>> +	kbuf.memsz += text_offset;
> >>> +
> >>> +	ret = kexec_add_buffer(&kbuf);
> >>> +	if (ret)
> >>> +		goto out;
> >>> +
> >>> +	image->arch.kern_segment = image->nr_segments - 1;
> >>
> >> You only seem to use kern_segment here, and in load_other_segments() called
> >> below. Could it not be a local variable passed in? Instead of arch-specific data
> >> we keep forever?
> > 
> > No, kern_segment is also used in load_other_segments() in machine_kexec_file.c.
> > To optimize memory hole allocation logic in locate_mem_hole_callback(),
> > we need to know the exact range of kernel image (start and end).
> 
> That's the second user. My badly-made point is one calls the other, but passes
> the data via some until-kexec lifetime struct. (its not important, just an
> indicator this worked differently in the past and hasn't been cleaned up).
> I meant something like [0].

OK, but instead of adding kern_seg, I want to change the interface to:

| extern int load_other_segments(struct kimage *image,
|		unsigned long kernel_load_addr, unsigned long kernel_size,
|		char *initrd, unsigned long initrd_len,
|		char *cmdline, unsigned long cmdline_len);

This way, we will in future be able to address an issue I mentioned in
my previous e-mail. (If we support vmlinux, the kernel occupies two segments
for text and data, respectively.)

Thanks,
-Takahiro AKASHI


> 
> Thanks,
> 
> James
> 
> 
> [0] a diff is worth a thousand words:
> --------------------%<--------------------
> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_
> kexec_file.c
> index 762f9102899c..c50ce844f09e 100644
> --- a/arch/arm64/kernel/machine_kexec_file.c
> +++ b/arch/arm64/kernel/machine_kexec_file.c
> @@ -325,11 +325,10 @@ static int prepare_elf_headers(void **addr, unsigned long *sz)
>         return ret;
>  }
> 
> -int load_other_segments(struct kimage *image,
> +int load_other_segments(struct kimage *image, struct kexec_segment *kern_seg,
>                         char *initrd, unsigned long initrd_len,
>                         char *cmdline, unsigned long cmdline_len)
>  {
> -       struct kexec_segment *kern_seg;
>         struct kexec_buf kbuf;
>         void *hdrs_addr;
>         unsigned long hdrs_sz;
> @@ -368,7 +367,6 @@ int load_other_segments(struct kimage *image,
>                                  image->arch.elf_load_addr, hdrs_sz, hdrs_sz);
>         }
> 
> -       kern_seg = &image->segment[image->arch.kern_segment];
>         kbuf.image = image;
>         /* not allocate anything below the kernel */
>         kbuf.buf_min = kern_seg->mem + kern_seg->memsz;
> diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
> index 891f2484969d..085cb69293ca 100644
> --- a/arch/arm64/include/asm/kexec.h
> +++ b/arch/arm64/include/asm/kexec.h
> @@ -173,8 +172,10 @@ static inline int arm64_header_check_pe_sig(const struct ar
> m64_image_header *h)
>  extern const struct kexec_file_ops kexec_image_ops;
> 
>  struct kimage;
> +struct kexec_segment;
> 
>  extern int load_other_segments(struct kimage *image,
> +               struct kexec_segment *kern_seg,
>                 char *initrd, unsigned long initrd_len,
>                 char *cmdline, unsigned long cmdline_len);
>  #endif
> diff --git a/arch/arm64/kernel/kexec_image.c b/arch/arm64/kernel/kexec_image.c
> index 7c11beefe65f..0e032d30a79c 100644
> --- a/arch/arm64/kernel/kexec_image.c
> +++ b/arch/arm64/kernel/kexec_image.c
> @@ -37,6 +37,7 @@ static void *image_load(struct kimage *image,
>                                 char *cmdline, unsigned long cmdline_len)
>  {
>         struct kexec_buf kbuf;
> +       struct kexec_segment *kern_seg;
>         struct arm64_image_header *h = (struct arm64_image_header *)kernel;
>         unsigned long text_offset;
>         int ret;
> @@ -65,17 +66,17 @@ static void *image_load(struct kimage *image,
>         if (ret)
>                 goto out;
> 
> -       image->arch.kern_segment = image->nr_segments - 1;
> -       image->segment[image->arch.kern_segment].mem += text_offset;
> -       image->segment[image->arch.kern_segment].memsz -= text_offset;
> -       image->start = image->segment[image->arch.kern_segment].mem;
> +       kern_seg = &image->segment[image->nr_segments - 1];
> +       kern_seg->mem += text_offset;
> +       kern_seg->memsz -= text_offset;
> +       image->start = kern_seg->mem;
> 
>         pr_debug("Loaded kernel at 0x%lx bufsz=0x%lx memsz=0x%lx\n",
> -                               image->segment[image->arch.kern_segment].mem,
> +                               kern_seg->mem,
>                                 kbuf.bufsz, kbuf.memsz);
> 
>         /* Load additional data */
> -       ret = load_other_segments(image, initrd, initrd_len,
> +       ret = load_other_segments(image, kern_seg, initrd, initrd_len,
>                                 cmdline, cmdline_len);
> 
>  out:
> --------------------%<--------------------