Re: [RFC PATCH v2 2/2] ELF: Add ELF program property parsing support

[Yu-cheng Yu]

On Mon, 2019-09-02 at 10:28 +0100, Dave Martin wrote:
> On Fri, Aug 30, 2019 at 06:03:27PM +0100, Yu-cheng Yu wrote:
> > On Fri, 2019-08-30 at 09:34 +0100, Dave Martin wrote:
> > > On Fri, Aug 30, 2019 at 06:37:45AM +0100, Kees Cook wrote:
> > > > On Fri, Aug 23, 2019 at 06:23:40PM +0100, Dave Martin wrote:
> > > > > ELF program properties will needed for detecting whether to enable
> > > > > optional architecture or ABI features for a new ELF process.
> > > > > 
> > > > > For now, there are no generic properties that we care about, so do
> > > > > nothing unless CONFIG_ARCH_USE_GNU_PROPERTY=y.
> > > > > 
> > > > > Otherwise, the presence of properties using the PT_PROGRAM_PROPERTY
> > > > > phdrs entry (if any), and notify each property to the arch code.
> > > > > 
> > > > > For now, the added code is not used.
> > > > > 
> > > > > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> > > > 
> > > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > 
> > > Thanks for the review.
> > > 
> > > Do you have any thoughts on Yu-Cheng Yu's comments?  It would be nice to
> > > early-terminate the scan if we can, but my feeling so far was that the
> > > scan is cheap, the number of properties is unlikely to be more than a
> > > smallish integer, and the code separation benefits of just calling the
> > > arch code for every property probably likely outweigh the costs of
> > > having to iterate over every property.  We could always optimise it
> > > later if necessary.
> > > 
> > > I need to double-check that there's no way we can get stuck in an
> > > infinite loop with the current code, though I've not seen it in my
> > > testing.  I should throw some malformed notes at it though.
> > 
> > Here is my arch_parse_elf_property() and objdump of the property.
> > The parser works fine.
> 
> [...]
> 
> > int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
> >           
> >                    bool compat, struct arch_elf_state *state)
> > {
> >         if (type
> > != GNU_PROPERTY_X86_FEATURE_1_AND)
> >                 return -ENOENT;
> 
> For error returns, I was following this convention:
> 
> 	EIO: invalid ELF file
> 
> 	ENOEXEC: valid ELF file, but we can't (or won't) support it
> 
> 	0: OK, or don't care

>From errno-base.h, EIO is for I/O error; ENOEXEC is for Exec format error.
Is this closer to what is happening?

> 
> This function gets called for every property, including properties that
> the arch code may not be interested in, so for properties you don't care
> about here you should return 0.

Yes.

> 
> > 
> >         if (datasz < sizeof(unsigned int))
> >                 return -ENOEXEC;
> 
> Should this be != ?
> 
> According to the draft x86-64 psABI spec [1],
> X86_PROPERTY_FEATURE_1_AND (and all properties based on
> GNU_PROPERTY_X86_UINT32_AND_LO) has data consisting of a single 4-byte
> unsigned integer.
> 
> >         state->gnu_property = *(unsigned int *)data;
> >         return 0;
> > }

Yes, I will change it.

Thanks,
Yu-cheng