Re: [PATCH] x86: allow i386 kexec to load x86_64 bzImage anywhere

[Kevin Mitchell]

On Mon, Aug 09, 2021 at 10:13:16AM +0800, Baoquan He wrote:
> On 08/06/21 at 07:21pm, Kevin Mitchell wrote:
> > In linux-5.2, the following commit allows the kdump kernel to be loaded
> > at a higher address than 896M
> > 
> > 9ca5c8e632ce ("x86/kdump: Have crashkernel=X reserve under 4G by
> > default")
> > 
> > While this limit does indeed seem unnecessary for x86_64 kernels, it
> > still is required to boot to or from i386 kernels. Therefore,
> > kexec-tools continues to enforce it when using the i386 bzImage loader.
> > 
> > However, the i386 bzImage loader may also be used to load an x86_64
> > kernel from i386 user space to be kexeced by an x86_64 kernel. In this
> > case, the limit was incorrectly enforced.
> 
> Are you doing kexec/kdump switching to x86_64 kernel on a i386 system?

We run an x86_64 kernel, but our initrd userspace in which we load the x86_64
kdump kernel is i386 to conserve space as many of our smaller devices are memory
constrained. We also need to fit this initrd into our coreboot SPI flash image
which is limited to a few megabytes.

> Could you tell more about your testing or product environment so that we
> know why we need to do that?

Previous to 9ca5c8e632ce, this worked without issue because the crashkernel area
was always reserved in a location that satisfied the limits defined in
kexec-bzImage.c even on an x86_64 kernel. Once we switched to
linux-5.10, we started seeing

Aboot# kexec --load-panic --initrd=initrd-i386-kdump
    --command-line="$crash_cmd_line" linux-x86_64-kdump
Could not find a free area of memory of 0x8000 bytes...
locate_hole failed

which is the result of hitting the kexec-bzImage.c limits. However, these appear
not to apply in the case of x86_64 kernel loaded from an x86_64 kernel even when
using an i386 kexec.

With this patch, I am able to load the kdump kernel into the new default
crashkernel location assigned by the linux-5.10 kernel (e.g., 1968MB, 3264MB)
and have it successfully kexeced when triggering a panic. This was tested on all
our current CPU platforms including both AMD (eKabini, Steppe Eagle, Crowned
Eagle, Merlin Falcon) and Intel (SandyBridge, Broadwell-DE) CPUs variously
running on between 4Gb - 64 Gb of RAM.

Conversely, I tried unconditionally removing the limits in kexec-bzImage.c, but
found that if either or both of the running or kdump kernels were i386 and the
crashkernel reservation was above the 896M limit (I had to force this as the
default location selected by the kernel is below this), the kexec would hang
indefinitely. Therefore, I have kept those limits in place when either kernel is
i386.

> AFAIK, we rarely kexec/kdump switch to
> x86_64 kenrel from a i386 kernel.
> 
> Thanks
> Baoquan
> 
> > 
> > This commit adds an additional check for an x86_64 image kexeced by an
> > x86_64 kernel in the i386 loader and bumps the limit to the maximum
> > addressable 4G in that case.
> > 
> > Signed-off-by: Kevin Mitchell <kevmitch@xxxxxxxxxx>
> > ---
> >  kexec/arch/i386/kexec-bzImage.c | 41 ++++++++++++++++++++++-----------
> >  1 file changed, 28 insertions(+), 13 deletions(-)
> > 
> > diff --git a/kexec/arch/i386/kexec-bzImage.c b/kexec/arch/i386/kexec-bzImage.c
> > index df8985d..7b8e36e 100644
> > --- a/kexec/arch/i386/kexec-bzImage.c
> > +++ b/kexec/arch/i386/kexec-bzImage.c
> > @@ -22,6 +22,7 @@
> >  #include <string.h>
> >  #include <stdlib.h>
> >  #include <errno.h>
> > +#include <limits.h>
> >  #include <sys/types.h>
> >  #include <sys/stat.h>
> >  #include <fcntl.h>
> > @@ -114,6 +115,7 @@ int do_bzImage_load(struct kexec_info *info,
> >  	struct entry32_regs regs32;
> >  	struct entry16_regs regs16;
> >  	unsigned int relocatable_kernel = 0;
> > +	unsigned int kernel64 = 0;
> >  	unsigned long kernel32_load_addr;
> >  	char *modified_cmdline;
> >  	unsigned long cmdline_end;
> > @@ -155,6 +157,13 @@ int do_bzImage_load(struct kexec_info *info,
> >  		dbgprintf("bzImage is relocatable\n");
> >  	}
> >  
> > +	if ((setup_header.protocol_version >= 0x020C) &&
> > +	    (info->kexec_flags & KEXEC_ARCH_X86_64) &&
> > +	    (setup_header.xloadflags & 1)) {
> > +		kernel64 = 1;
> > +		dbgprintf("loading x86_64 bzImage from an x86_64 kernel\n");
> > +	}
> > +
> >  	/* Can't use bzImage for crash dump purposes with real mode entry */
> >  	if((info->kexec_flags & KEXEC_ON_CRASH) && real_mode_entry) {
> >  		fprintf(stderr, "Can't use bzImage for crash dump purposes"
> > @@ -197,17 +206,17 @@ int do_bzImage_load(struct kexec_info *info,
> >  	/* Load the trampoline.  This must load at a higher address
> >  	 * than the argument/parameter segment or the kernel will stomp
> >  	 * it's gdt.
> > -	 *
> > -	 * x86_64 purgatory code has got relocations type R_X86_64_32S
> > -	 * that means purgatory got to be loaded within first 2G otherwise
> > -	 * overflow takes place while applying relocations.
> >  	 */
> > -	if (!real_mode_entry && relocatable_kernel)
> > +	if (!real_mode_entry && relocatable_kernel) {
> > +		/* x86_64 purgatory could be anywhere */
> > +		unsigned long purg_max_addr = kernel64 ? ULONG_MAX : 0x7fffffff;
> > +
> >  		elf_rel_build_load(info, &info->rhdr, purgatory, purgatory_size,
> > -					0x3000, 0x7fffffff, -1, 0);
> > -	else
> > +					0x3000, purg_max_addr, -1, 0);
> > +	} else {
> >  		elf_rel_build_load(info, &info->rhdr, purgatory, purgatory_size,
> >  					0x3000, 640*1024, -1, 0);
> > +	}
> >  	dbgprintf("Loaded purgatory at addr 0x%lx\n", info->rhdr.rel_addr);
> >  
> >  	/* The argument/parameter segment */
> > @@ -277,14 +286,20 @@ int do_bzImage_load(struct kexec_info *info,
> >  	if (real_mode->protocol_version >=0x0205 && relocatable_kernel) {
> >  		/* Relocatable bzImage */
> >  		unsigned long kern_align = real_mode->kernel_alignment;
> > -		unsigned long kernel32_max_addr = DEFAULT_BZIMAGE_ADDR_MAX;
> > +		unsigned long kernel_max_addr = DEFAULT_BZIMAGE_ADDR_MAX;
> >  
> > -		if (kernel32_max_addr > real_mode->initrd_addr_max)
> > -			kernel32_max_addr = real_mode->initrd_addr_max;
> > +		/*
> > +		 * x86_64 kernels can be kexeced by an x86_64 kernel
> > +		 * from any addressable location
> > +		 */
> > +		if (kernel64)
> > +			kernel_max_addr = ULONG_MAX;
> > +		else if (kernel_max_addr > real_mode->initrd_addr_max)
> > +			kernel_max_addr = real_mode->initrd_addr_max;
> >  
> >  		kernel32_load_addr = add_buffer(info, kernel + kern16_size,
> >  						size, size, kern_align,
> > -						0x100000, kernel32_max_addr,
> > +						0x100000, kernel_max_addr,
> >  						1);
> >  	}
> >  	else {
> > @@ -296,9 +311,9 @@ int do_bzImage_load(struct kexec_info *info,
> >  	dbgprintf("Loaded 32bit kernel at 0x%lx\n", kernel32_load_addr);
> >  
> >  	/* Tell the kernel what is going on */
> > -	setup_linux_bootloader_parameters(info, real_mode, setup_base,
> > +	setup_linux_bootloader_parameters_high(info, real_mode, setup_base,
> >  		kern16_size_needed, command_line, command_line_len,
> > -		initrd, initrd_len);
> > +		initrd, initrd_len, kernel64); /* put x86_64 initrd high too */
> >  
> >  	if (real_mode_entry && real_mode->protocol_version >= 0x0201) {
> >  		real_mode->loader_flags |= 0x80; /* CAN_USE_HEAP */
> > -- 
> > 2.32.0
> > 
>