Re: [PATCH] binfmt_elf: Extract .note.gnu.property from an ELF file

[Yu-cheng Yu]

On Thu, 2019-05-02 at 15:29 +0100, Dave Martin wrote:
> On Thu, May 02, 2019 at 12:10:04PM +0100, Dave Martin wrote:
> > On Wed, May 01, 2019 at 02:12:17PM -0700, Yu-cheng Yu wrote:
> 
> [...]
> 
> > > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c > > index
> > > 7d09d125f148..40aa4a4fd64d 100644
> > > --- a/fs/binfmt_elf.c
> > > +++ b/fs/binfmt_elf.c
> > > @@ -1076,6 +1076,19 @@ static int load_elf_binary(struct linux_binprm
> > > *bprm)
> > >  		goto out_free_dentry;
> > >  	}
> > >  
> > > +	if (interpreter) {
> > > +		retval = arch_setup_property(&loc->interp_elf_ex,
> > > +					     interp_elf_phdata,
> > > +					     interpreter, true);
> > > +	} else {
> > > +		retval = arch_setup_property(&loc->elf_ex,
> > > +					     elf_phdata,
> > > +					     bprm->file, false);
> > > +	}
> 
> This will be too late for arm64, since we need to twiddle the mmap prot
> flags for the executable's pages based on the detected properties.
> 
> Can we instead move this much earlier, letting the arch code stash
> something in arch_state that can be consumed later on?
> 
> This also has the advantage that we can report errors to the execve()
> caller before passing the point of no return (i.e., flush_old_exec()).

I will look into that.

> 
> [...]
> 
> > > diff --git a/fs/gnu_property.c b/fs/gnu_property.c
> 
> [...]
> 
> > > +int get_gnu_property(void *ehdr_p, void *phdr_p, struct file *f,
> > > +		     u32 pr_type, u32 *property)
> > > +{
> > > +	struct elf64_hdr *ehdr64 = ehdr_p;
> > > +	int err = 0;
> > > +
> > > +	*property = 0;
> > > +
> > > +	if (ehdr64->e_ident[EI_CLASS] == ELFCLASS64) {
> > > +		struct elf64_phdr *phdr64 = phdr_p;
> > > +
> > > +		err = scan_segments_64(f, phdr64, ehdr64->e_phnum,
> > > +				       pr_type, property);
> > > +		if (err < 0)
> > > +			goto out;
> > > +	} else {
> > > +#ifdef CONFIG_COMPAT
> > > +		struct elf32_hdr *ehdr32 = ehdr_p;
> > > +
> > > +		if (ehdr32->e_ident[EI_CLASS] == ELFCLASS32) {
> > > +			struct elf32_phdr *phdr32 = phdr_p;
> > > +
> > > +			err = scan_segments_32(f, phdr32, ehdr32-
> > > >e_phnum,
> > > +					       pr_type, property);
> > > +			if (err < 0)
> > > +				goto out;
> > > +		}
> > > +#else
> > > +	WARN_ONCE(1, "Exec of 32-bit app, but CONFIG_COMPAT is not
> > > enabled.\n");
> > > +	return -ENOTSUPP;
> > > +#endif
> > > +	}
> 
> We have already made a ton of assumptions about the ELF class by this
> point, and we don't seem to check it explicitly elsewhere, so it is a
> bit weird to police it specifically here.
> 
> Can we simply pass the assumed ELF class as a parameter instead?

Yes.

Yu-cheng