Re: [PATCH] Only search page_offset_base when KASLR mm code is compiled in

[Baoquan He <bhe@redhat.com>]

On 02/28/17 at 02:06pm, Dave Young wrote:
> On 02/24/17 at 06:38pm, Baoquan He wrote:
> > Otherwise it will print false positve message as below. So add
> > a check when do the search.
> > 
> >     "Cannot get kernel page_offset_base symbol address"
> > 
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > ---
> >  kexec/arch/i386/crashdump-x86.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
> > index 88aeee3..b2c1be5 100644
> > --- a/kexec/arch/i386/crashdump-x86.c
> > +++ b/kexec/arch/i386/crashdump-x86.c
> > @@ -150,8 +150,6 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
> >  	off_t size;
> >  	uint32_t elf_flags = 0;
> >  	uint64_t stext_sym;
> > -	const unsigned long long pud_mask = ~((1 << 30) - 1);
> > -	unsigned long long vaddr, lowest_vaddr = 0;
> >  
> >  	if (elf_info->machine != EM_X86_64)
> >  		return 0;
> > @@ -181,8 +179,11 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
> >  
> >  	end_phdr = &ehdr.e_phdr[ehdr.e_phnum];
> >  
> > +#ifdef CONFIG_RANDOMIZE_MEMORY
> 
> It does not work in userspace code.

Could be it only works when make header and install. Otherwise it can't
be seen by user space. I built a rhel kernel package and test passed.

Anyway, since it doesn't work in all cases, I have made another way to
fix it, just take out the error message and put it where get_kernel_sym
is called.

> 
> Maybe mute the error message or move it to kexec --debug is more
> reasonable..
> 
> >  	/* Search for the real PAGE_OFFSET when KASLR memory randomization
> >  	 * is enabled */
> > +	const unsigned long long pud_mask = ~((1 << 30) - 1);
> > +	unsigned long long vaddr, lowest_vaddr = 0;
> >  	if (get_kernel_sym("page_offset_base") != 0) {
> >  		for(phdr = ehdr.e_phdr; phdr != end_phdr; phdr++) {
> >  			if (phdr->p_type == PT_LOAD) {
> > @@ -194,6 +195,7 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
> >  		if (lowest_vaddr != 0)
> >  			elf_info->page_offset = lowest_vaddr;
> >  	}
> > +#endif
> >  
> >  	/* Traverse through the Elf headers and find the region where
> >  	 * _stext symbol is located in. That's where kernel is mapped */
> > -- 
> > 2.5.5
> > 
> > 
> > _______________________________________________
> > kexec mailing list
> > kexec@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/kexec
> 
> Thanks
> Dave
> 
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> kexec@lists.infradead.org
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