Re: [PATCH dwarves] dwarf_loader: Handle DW_AT_location attrs containing DW_OP_plus_uconst

[Yao Zi <me@ziyao.cc>]

Hi Yonghong,

Sorry for the late reply,

On Wed, Dec 03, 2025 at 04:46:20PM -0800, Yonghong Song wrote:
> 
> 
> On 11/29/25 7:21 PM, Yao Zi wrote:

...

> > diff --git a/dwarf_loader.c b/dwarf_loader.c
> > index 79be3f516a26..635015676389 100644
> > --- a/dwarf_loader.c
> > +++ b/dwarf_loader.c
> > @@ -708,6 +708,11 @@ static enum vscope dwarf__location(Dwarf_Die *die, uint64_t *addr, struct locati
> >   		case DW_OP_addrx:
> >   			scope = VSCOPE_GLOBAL;
> >   			*addr = expr[0].number;
> > +
> > +			if (location->exprlen == 2 &&
> > +			    expr[1].atom == DW_OP_plus_uconst)
> > +				addr += expr[1].number;
> 
> This does not work. 'addr' is the parameter and the above new 'addr' value won't
> pass back to caller so the above is effectively a noop.

Oops, this is a silly problem.

> I think we need to add an additional parameter to pass the 'expr[1].number' back
> to the caller, e.g.,

However, I don't think it's necessary. See my explanation below,

> static enum vscope dwarf__location(Dwarf_Die *die, uint64_t *addr, uint32_t *offset, struct location *location) { ... }
> 
> and
> 
>    in the above
>        *offset = expr[1].number.
> 
> Now the caller has the following information:
>   . The deference of *addr stores the index to .debug_addr

No, dwarf__location() invokes attr_location(), which calls
dwarf_getlocation() and dwarf_formaddr(), the latter already performs a
lookup in .debug_addr[1], so what is stored in *addr is right the symbol
address.

Thus I think it's enough to keep the signature, but add the offset to
*addr.

>   . The offset to the address in .debug_addr
> and the final address will be debug_addr[*addr] + offset.
> 
> > +
> >   			break;
> >   		case DW_OP_reg1 ... DW_OP_reg31:
> >   		case DW_OP_breg0 ... DW_OP_breg31:
> 

Thanks for your review, I'll soon send a patch with the missing pointer
dereference to addr added.

Best regards,
Yao Zi

[1]: https://github.com/sourceware-org/elfutils/blob/67199e1c974db37f2bd200dcca7d7103f42ed06e/libdw/dwarf_formaddr.c#L37-L77