Re: [PATCH v5 02/13] x86/kexec: refactor for kernel/Kconfig.kexec

[Eric DeVolder <eric.devolder@xxxxxxxxxx>]

On 7/7/23 00:58, Christophe Leroy wrote:
> Le 07/07/2023 à 00:20, Eric DeVolder a écrit :
> > The kexec and crash kernel options are provided in the common
> > kernel/Kconfig.kexec. Utilize the common options and provide
> > the ARCH_SUPPORTS_ and ARCH_SELECTS_ entries to recreate the
> > equivalent set of KEXEC and CRASH options.
>
>
> Why do you need to duplicate the ARCH_SELECTS_ entries in each
> architecture ?
>
> Why not define them in arch/Kconfig then select if from each architecture ?
>
> For instance here for x86 for ARCH_SELECTS_KEXEC_FILE all you'll have to
> do is:
> 	select ARCH_SELECTS_KEXEC_FILE if KEXEC_FILE
> 	select HAVE_IMA_KEXEC if IMA && KEXEC_FILE
>
>
> Christophe

Hi Christophe, thanks for looking at this!
The ARCH_SELECTS_ is where the "customization" is performed for that option. The customization is 
putting in place the differences that the arch had from the common option.

For example, with ARCH_SELECTS_KEXEC_FILE, here are all the uses:

x86:
    select HAVE_IMA_KEXEC if IMA

arm64:
    select HAVE_IMA_KEXEC if IMA

powerpc:
    select KEXEC_ELF
    select HAVE_IMA_KEXEC if IMA

riscv:
    select HAVE_IMA_KEXEC if IMA
    select KEXEC_ELF

And there are archs (parisc, riscv, s390) that support KEXEC_FILE that do not need 
ARCH_SELECTS_KEXEC_FILE.

So there isn't a clear path for optimizing the IMA statement, in this example, that I'm aware of.

But perhaps I don't quite understand the technique you are suggesting, either...

eric



> > Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> > ---
> >    arch/x86/Kconfig | 92 ++++++++++--------------------------------------
> >    1 file changed, 19 insertions(+), 73 deletions(-)
> >
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 7422db409770..9767a343f7c2 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -2040,88 +2040,34 @@ config EFI_RUNTIME_MAP
> >    
> >    source "kernel/Kconfig.hz"
> >    
> > -config KEXEC
> > -	bool "kexec system call"
> > -	select KEXEC_CORE
> > -	help
> > -	  kexec is a system call that implements the ability to shutdown your
> > -	  current kernel, and to start another kernel.  It is like a reboot
> > -	  but it is independent of the system firmware.   And like a reboot
> > -	  you can start any kernel with it, not just Linux.
> > -
> > -	  The name comes from the similarity to the exec system call.
> > -
> > -	  It is an ongoing process to be certain the hardware in a machine
> > -	  is properly shutdown, so do not be surprised if this code does not
> > -	  initially work for you.  As of this writing the exact hardware
> > -	  interface is strongly in flux, so no good recommendation can be
> > -	  made.
> > -
> > -config KEXEC_FILE
> > -	bool "kexec file based system call"
> > -	select KEXEC_CORE
> > -	select HAVE_IMA_KEXEC if IMA
> > -	depends on X86_64
> > -	depends on CRYPTO=y
> > -	depends on CRYPTO_SHA256=y
> > -	help
> > -	  This is new version of kexec system call. This system call is
> > -	  file based and takes file descriptors as system call argument
> > -	  for kernel and initramfs as opposed to list of segments as
> > -	  accepted by previous system call.
> > +config ARCH_SUPPORTS_KEXEC
> > +	def_bool y
> >    
> > -config ARCH_HAS_KEXEC_PURGATORY
> > -	def_bool KEXEC_FILE
> > +config ARCH_SUPPORTS_KEXEC_FILE
> > +	def_bool X86_64 && CRYPTO && CRYPTO_SHA256
> >    
> > -config KEXEC_SIG
> > -	bool "Verify kernel signature during kexec_file_load() syscall"
> > +config ARCH_SELECTS_KEXEC_FILE
> > +	def_bool y
> >    	depends on KEXEC_FILE
> > -	help
> > +	select HAVE_IMA_KEXEC if IMA
> >    
> > -	  This option makes the kexec_file_load() syscall check for a valid
> > -	  signature of the kernel image.  The image can still be loaded without
> > -	  a valid signature unless you also enable KEXEC_SIG_FORCE, though if
> > -	  there's a signature that we can check, then it must be valid.
> > +config ARCH_HAS_KEXEC_PURGATORY
> > +	def_bool KEXEC_FILE
> >    
> > -	  In addition to this option, you need to enable signature
> > -	  verification for the corresponding kernel image type being
> > -	  loaded in order for this to work.
> > +config ARCH_SUPPORTS_KEXEC_SIG
> > +	def_bool y
> >    
> > -config KEXEC_SIG_FORCE
> > -	bool "Require a valid signature in kexec_file_load() syscall"
> > -	depends on KEXEC_SIG
> > -	help
> > -	  This option makes kernel signature verification mandatory for
> > -	  the kexec_file_load() syscall.
> > +config ARCH_SUPPORTS_KEXEC_SIG_FORCE
> > +	def_bool y
> >    
> > -config KEXEC_BZIMAGE_VERIFY_SIG
> > -	bool "Enable bzImage signature verification support"
> > -	depends on KEXEC_SIG
> > -	depends on SIGNED_PE_FILE_VERIFICATION
> > -	select SYSTEM_TRUSTED_KEYRING
> > -	help
> > -	  Enable bzImage signature verification support.
> > +config ARCH_SUPPORTS_KEXEC_BZIMAGE_VERIFY_SIG
> > +	def_bool y
> >    
> > -config CRASH_DUMP
> > -	bool "kernel crash dumps"
> > -	depends on X86_64 || (X86_32 && HIGHMEM)
> > -	help
> > -	  Generate crash dump after being started by kexec.
> > -	  This should be normally only set in special crash dump kernels
> > -	  which are loaded in the main kernel with kexec-tools into
> > -	  a specially reserved region and then later executed after
> > -	  a crash by kdump/kexec. The crash dump kernel must be compiled
> > -	  to a memory address not used by the main kernel or BIOS using
> > -	  PHYSICAL_START, or it must be built as a relocatable image
> > -	  (CONFIG_RELOCATABLE=y).
> > -	  For more details see Documentation/admin-guide/kdump/kdump.rst
> > +config ARCH_SUPPORTS_KEXEC_JUMP
> > +	def_bool y
> >    
> > -config KEXEC_JUMP
> > -	bool "kexec jump"
> > -	depends on KEXEC && HIBERNATION
> > -	help
> > -	  Jump between original kernel and kexeced kernel and invoke
> > -	  code in physical address mode via KEXEC
> > +config ARCH_SUPPORTS_CRASH_DUMP
> > +	def_bool X86_64 || (X86_32 && HIGHMEM)
> >    
> >    config PHYSICAL_START
> >    	hex "Physical address where the kernel is loaded" if (EXPERT || CRASH_DUMP)