Re: [PATCH v3] riscv: elf: add .riscv.attributes parsing

[Vineet Gupta]

On 1/19/23 12:33, Jessica Clarke wrote:
> > +
> > +/*
> > + * Parse a single elf attribute.
> > + */
> > +static int rv_parse_elf_attr_safe(unsigned char **dpp, unsigned char *p_end)
> > +{
> > +	unsigned char *p = *dpp;
> > +	unsigned char *str;
> > +	u64 tag, val;
> > +	u32 s_len;
> > +
> > +	if (decode_uleb128_safe(&p, &tag, p_end))
> > +		goto bad_attr;
> > +
> > +	switch (tag) {
> > +	case RV_ATTR_TAG_stack_align:
> > +		if (decode_uleb128_safe(&p, &val, p_end))
> > +			goto bad_attr;
> > +		if (!ELF_ATTR_TAG_EVEN(tag))
> > +			goto bad_attr;
> Huh? You just checked it against a constant so you know exactly what it
> is. This is just Static_assert(RV_ATTR_TAG_stack_align % 2 == 0) but at
> run time. And you know that’s going to be the case, because the spec is
> self-consistent by design (wouldn’t be a worthwhile spec otherwise).

Makes sense.

> > +		rv_elf_attr_int(tag, val);
> > +		break;
> > +
> > +	case RV_ATTR_TAG_unaligned_access:
> > +		if (decode_uleb128_safe(&p, &val, p_end))
> > +			goto bad_attr;
> > +		if (!ELF_ATTR_TAG_EVEN(tag))
> > +			goto bad_attr;
> > +		rv_elf_attr_int(tag, val);
> > +		break;
> > +
> > +	case RV_ATTR_TAG_arch:
> > +		if (ELF_ATTR_TAG_EVEN(tag))
> > +			goto bad_attr;
> > +		str = p;
> > +		s_len = strnlen(p, p_end - p) + 1;
> > +		if ((p + s_len) > p_end)
> > +			goto bad_attr;
> > +		p += s_len;
> > +		rv_elf_attr_str(tag, str);
> > +		break;
> > +
> > +	default:
> The whole point of the even/odd split is so that when you *don’t* know
> what the tag means you can still decode its value and thus know how to
> skip past it. That is, *here* is where you need to be checking
> even/odd, and deciding whether to treat it as a string or a ULEB128,

I see the point. We can ignore the specific tags and just treat odd and 
even tags as string and int respectively.
And keep a loose check of the known tags vs. unknown.

> which is why I annotated *here* not one of the other case labels before.

OK. I really need to pay attention to what and where to your comments :-)


> > +	memset(buf, 0, RV_ATTR_SEC_SZ);
> > +	pos = phdr->p_offset;
> > +	n = kernel_read(f, &buf, phdr->p_filesz, &pos);
> > +
> > +	if (n <= 0)
> > +		return -EIO;
> 0 should be ENOEXEC not EIO? And surely in the < 0 case you want to be
> forwarding on the exact error from kernel_read, not squashing it into
> EIO?

Right.

> > +/*
> > + * Hook invoked by generic elf loader to parse riscv specific elf segments.
> > + */
> > +int arch_elf_pt_proc(void *_ehdr, void *_phdr, struct file *elf,
> > +		     bool is_interp, struct arch_elf_state *state)
> > +{
> > +	struct elf_phdr *phdr = _phdr;
> > +	int ret = 0;
> > +
> > +	if (phdr->p_type == PT_RISCV_ATTRIBUTES && !is_interp)
> Both the executable and its interpreter matter.

OK.

Thx,
-Vineet