Re: [Xen-devel] [PATCH v9 11/27] xsplice: Implement payload loading

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> > +static int move_payload(struct payload *payload, struct xsplice_elf *elf)
> > +{
.. snip..
> > +    /* Compute size of different regions. */
> > +    for ( i = 1; i < elf->hdr->e_shnum; i++ )
> > +    {
> > +        if ( (elf->sec[i].sec->sh_flags & (SHF_ALLOC|SHF_EXECINSTR)) ==
> > +             (SHF_ALLOC|SHF_EXECINSTR) )
> > +            calc_section(&elf->sec[i], &payload->text_size, &offset[i]);
> 
> This silently accepts writable text sections, yet the portion of the
> memory this gets placed in will be mapped RX.

I am not sure I follow. We only accept if sh_flags have AX. Not WAX?
How am I accepting writable text sections?
> 
> > +        else if ( (elf->sec[i].sec->sh_flags & SHF_ALLOC) &&
> > +                  !(elf->sec[i].sec->sh_flags & SHF_EXECINSTR) &&
> > +                  (elf->sec[i].sec->sh_flags & SHF_WRITE) )
> > +            calc_section(&elf->sec[i], &payload->rw_size, &offset[i]);
> > +        else if ( (elf->sec[i].sec->sh_flags & SHF_ALLOC) &&
> > +                  !(elf->sec[i].sec->sh_flags & SHF_EXECINSTR) &&
> > +                  !(elf->sec[i].sec->sh_flags & SHF_WRITE) )
> > +            calc_section(&elf->sec[i], &payload->ro_size, &offset[i]);
> > +        else if ( !elf->sec[i].sec->sh_flags ||
> > +                  (elf->sec[i].sec->sh_flags & SHF_EXECINSTR) ||
> > +                  (elf->sec[i].sec->sh_flags & SHF_MASKPROC) )
> > +            /* Do nothing.*/;
> > +        else if ( (elf->sec[i].sec->sh_flags & SHF_ALLOC) &&
> > +                  (elf->sec[i].sec->sh_type == SHT_NOBITS) )
> > +        {
> > +            dprintk(XENLOG_DEBUG, XSPLICE "%s: Not supporting %s 
> > section!\n",
> > +                    elf->name, elf->sec[i].name);
> > +            rc = -EOPNOTSUPP;
> > +            goto out;
> > +        }
> 
> I saw this in the changelog, but I don't really understand these last
> two conditionals. Wouldn't you want to bail on _any_ sections which

The first (/Do nothing/) is for sections such as .rela.* (which we can
ditch after we are done), .symtab, .strtab (for which in later patches in
build_symbol_table construct a copy), and:

[ 1] .note.gnu.build-i NOTE 0000000000000000  00000040
       0000000000000024  0000000000000000   A       0     0     4

which value we just copy in struct payload->id.
(also in later patch).
> have SHF_ALLOC set but don't get mapped to one of the three
> blocks? And wouldn't you (silently) ignore any sections with SHF_ALLOC
> clear?

Correct, such as:
 [29] .shstrtab         STRTAB           0000000000000000  000002fe
       0000000000000143  0000000000000000           0     0     1

I've update the comments to be more clear.

..snip..
> > +int xsplice_elf_resolve_symbols(struct xsplice_elf *elf)
> > +{
> > +    unsigned int i;
> > +    int rc = 0;
> > +
> > +    ASSERT(elf->sym);
> > +
> > +    for ( i = 1; i < elf->nsym; i++ )
> > +    {
> > +        unsigned int idx = elf->sym[i].sym->st_shndx;
> > +        Elf_Sym *sym = (Elf_Sym *)elf->sym[i].sym;
> 
> Well, I admit that this is the more straightforward solution, but it
> opens up all of what sym points to for writing. I.e. I'd have
> considered it much better to really only do the casting away of
> const in the one spot where you need it (see below).

OK. That may become a bit cumbersome. We would have in the later
patches (xsplice,symbols: Implement symbol name resolution on addres)
the SHN_UNDEF doing symbol lookup. And that one tries to set
sym->st_value twice.

I can certainly cast it twice there, and then once in the default
case if you would like.

> 
> > +        switch ( idx )
> > +        {
> > +        case SHN_COMMON:
> > +            dprintk(XENLOG_ERR, XSPLICE "%s: Unexpected common symbol: 
> > %s\n",
> > +                    elf->name, elf->sym[i].name);
> > +            rc = -EINVAL;
> > +            break;
> > +
> > +        case SHN_UNDEF:
> > +            dprintk(XENLOG_ERR, XSPLICE "%s: Unknown symbol: %s\n",
> > +                    elf->name, elf->sym[i].name);
> > +            rc = -ENOENT;
> > +            break;
..snip..
> > +        default:

..snip..
> > +
> > +            sym->st_value += (unsigned long)elf->sec[idx].load_addr;
> 
> *(<type> *)&sym->st_value += ...

Right, so I can cast it to a non-const and write to it. Keep in mind
that in this patch it is only one place, but in further I would have
to do this twice.

What is your preference?
> 

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