Re: [XEN v6] xen/arm: Probe the load/entry point address of an uImage correctly

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 25/01/2023 21:06, Stefano Stabellini wrote:
> On Wed, 25 Jan 2023, Ayan Kumar Halder wrote:
> > Currently, kernel_uimage_probe() does not read the load/entry point address
> > set in the uImge header. Thus, info->zimage.start is 0 (default value). This
> > causes, kernel_zimage_place() to treat the binary (contained within uImage)
> > as position independent executable. Thus, it loads it at an incorrect
> > address.
> >
> > The correct approach would be to read "uimage.load" and set
> > info->zimage.start. This will ensure that the binary is loaded at the
> > correct address. Also, read "uimage.ep" and set info->entry (ie kernel entry
> > address).
> >
> > If user provides load address (ie "uimage.load") as 0x0, then the image is
> > treated as position independent executable. Xen can load such an image at
> > any address it considers appropriate. A position independent executable
> > cannot have a fixed entry point address.
> >
> > This behavior is applicable for both arm32 and arm64 platforms.
> >
> > Earlier for arm32 and arm64 platforms, Xen was ignoring the load and entry
> > point address set in the uImage header. With this commit, Xen will use them.
> > This makes the behavior of Xen consistent with uboot for uimage headers.
> >
> > Users who want to use Xen with statically partitioned domains, can provide
> > non zero load address and entry address for the dom0/domU kernel. It is
> > required that the load and entry address provided must be within the memory
> > region allocated by Xen.
> >
> > A deviation from uboot behaviour is that we consider load address == 0x0,
> > to denote that the image supports position independent execution. This
> > is to make the behavior consistent across uImage and zImage.
> >
> > Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
> > ---
> >
> > Changes from v1 :-
> > 1. Added a check to ensure load address and entry address are the same.
> > 2. Considered load address == 0x0 as position independent execution.
> > 3. Ensured that the uImage header interpretation is consistent across
> > arm32 and arm64.
> >
> > v2 :-
> > 1. Mentioned the change in existing behavior in booting.txt.
> > 2. Updated booting.txt with a new section to document "Booting Guests".
> >
> > v3 :-
> > 1. Removed the constraint that the entry point should be same as the load
> > address. Thus, Xen uses both the load address and entry point to determine
> > where the image is to be copied and the start address.
> > 2. Updated documentation to denote that load address and start address
> > should be within the memory region allocated by Xen.
> > 3. Added constraint that user cannot provide entry point for a position
> > independent executable (PIE) image.
> >
> > v4 :-
> > 1. Explicitly mentioned the version in booting.txt from when the uImage
> > probing behavior has changed.
> > 2. Logged the requested load address and entry point parsed from the uImage
> > header.
> > 3. Some style issues.
> >
> > v5 :-
> > 1. Set info->zimage.text_offset = 0 in kernel_uimage_probe().
> > 2. Mention that if the kernel has a legacy image header on top of 
> > zImage/zImage64
> > header, then the attrbutes from legacy image header is used to determine the 
> > load
> > address, entry point, etc. Thus, zImage/zImage64 header is effectively ignored.
> >
> > This is true because Xen currently does not support recursive probing of kernel
> > headers ie if uImage header is probed, then Xen will not attempt to see if there
> > is an underlying zImage/zImage64 header.
> >
> >   docs/misc/arm/booting.txt         | 30 ++++++++++++++++
> >   xen/arch/arm/include/asm/kernel.h |  2 +-
> >   xen/arch/arm/kernel.c             | 58 +++++++++++++++++++++++++++++--
> >   3 files changed, 86 insertions(+), 4 deletions(-)
> >
> > diff --git a/docs/misc/arm/booting.txt b/docs/misc/arm/booting.txt
> > index 3e0c03e065..1837579aef 100644
> > --- a/docs/misc/arm/booting.txt
> > +++ b/docs/misc/arm/booting.txt
> > @@ -23,6 +23,32 @@ The exceptions to this on 32-bit ARM are as follows:
> >   
> >   There are no exception on 64-bit ARM.
> >   
> > +Booting Guests
> > +--------------
> > +
> > +Xen supports the legacy image header[3], zImage protocol for 32-bit
> > +ARM Linux[1] and Image protocol defined for ARM64[2].
> > +
> > +Until Xen 4.17, in case of legacy image protocol, Xen ignored the load
> > +address and entry point specified in the header. This has now changed.
> > +
> > +Now, it loads the image at the load address provided in the header.
> > +And the entry point is used as the kernel start address.
> > +
> > +A deviation from uboot is that, Xen treats "load address == 0x0" as
> > +position independent execution (PIE). Thus, Xen will load such an image
> > +at an address it considers appropriate. Also, user cannot specify the
> > +entry point of a PIE image since the start address cennot be
> > +predetermined.
> > +
> > +Users who want to use Xen with statically partitioned domains, can provide
> > +the fixed non zero load address and start address for the dom0/domU kernel.
> > +The load address and start address specified by the user in the header must
> > +be within the memory region allocated by Xen.
> > +
> > +Also, it is to be noted that if user provides the legacy image header on top of
> > +zImage or Image header, then Xen uses the attrbutes of legacy image header only
>                                               ^ attributes                    ^ 
> remove only
>
> > +to determine the load address, entry point, etc.
> Also add:
>
> """
> Known limitation: compressed kernels with a uboot headers are not
> working.
> """
I am not entirely sure where you want this sentence to be added. So...

> These few minor changes to the documentation can be done on commit:
... can you take care of committing it?

Cheers,

--
Julien Grall