Re: [PATCH] build: avoid absolute paths in executables

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.09.2025 18:40, Andrew Cooper wrote:
> On 03/09/2025 5:12 pm, Jan Beulich wrote:
>> On 03.09.2025 17:26, Andrew Cooper wrote:
>>> On 03/09/2025 4:13 pm, Jan Beulich wrote:
>>>> --- a/xen/Makefile
>>>> +++ b/xen/Makefile
>>>> @@ -448,6 +448,8 @@ LDFLAGS-$(CONFIG_CC_IS_CLANG) += -plugin
>>>>  endif
>>>>  
>>>>  ifdef building_out_of_srctree
>>>> +    CFLAGS += $(call cc-option,$(CC),-ffile-prefix-map=$(srctree)/=, \
>>>> +                                     -fdebug-prefix-map=$(srctree)/=)
>>>>      CFLAGS += -I$(objtree)/include
>>>>      CFLAGS += -I$(objtree)/arch/$(SRCARCH)/include
>>>>  endif
>>> We do want to be taking a change like this, but it's also definitely not
>>> limited to out-of-tree builds.  I have full paths embedded even for
>>> in-tree builds.
>> In xen-syms I see only two full paths - in debug info, supplying the base
>> path to the tree.

What I'm missing from your reply is clarification whether the mentioned
instances are indeed the only ones you see, or whether there's more in
what you have (and what I'm not seeing for whatever reason).

>> That's okay to stay imo.
> 
> Not for reducible builds it's not.

Yes, I realized this later.

However, I can see benefits to both: When one wants reproducible builds,
no absolute path whatsoever should remain. In other (debugging) cases
having a reference to the root of what everything else is relative to
might be helpful. So whether to replace these remaining instances may
want to be configurable (in turn making it necessary to deal with that
independently for xen/ and tools/; for xen/ that would be a Kconfig
option dependent upon DEBUG_INFO=y).

In out-of-tree builds similar references exist to the build tree root.
Once we zap both, the result is at risk of being ambiguous. I wonder
whether it would be possible (supported by consumers) to replace both
references by something macro-like (along the lines of $SRC/ and $BLD/).

>> In xen.efi I see a few hundred, but they're all the same as above. As I
>> learned earlier today, SHF_MERGE processing isn't invoked when linking
>> ELF objects into a PE binary.
>>
>>> To be useful, it wants to apply to everything, not just the hypervisor,
>>> so does want to be in the top level Config.mk.
>> As per my first remark then. But no, I meanwhile realized that this can't
>> go in Config.mk: For the hypervisor we want to use $(srctree), i.e.
>> including the leaf /xen referencing the xen/ subtree. I expect that for
>> e.g. tools/libs/ we'd want something similar - eliminate the entire path
>> up to the base of the component's source dir. So it will need to be
>> piecemeal.
> 
> Relative to the root of xen.git (or the source tarball) is the only
> sensible option.  Anything else is intentionally misleading.

I disagree. In-tree builds record things downward from xen/ only. So should
out-of-tree builds do. Every individual binary (i.e. including all the tools/
ones) has no need to record anything more than is necessary to unambiguously
identify the source files. In particular us bundling hypervisor, toolstack,
and stubdom (and there we expand various external packages) in a single
repo / tarball is an artifact, not how things normally would be arranged.

> In fact, Marek had a more-correct form of this patch in
> https://lore.kernel.org/xen-devel/0370c0eb1fd9ac00acab016792132fa0b943d384.1742317309.git-series.marmarek@xxxxxxxxxxxxxxxxxxxxxx/T/#u
> which seems to be waiting on you to reply.

I can't spot anything expecting my reply. What I can spot is a promise to
submit a v2. And, having entirely forgotten that there already was an
attempt, I only now realize why the options coming into play seemed
somewhat familiar.

Jan