Debug info linking is currently broken due to how we handle reading and laying out non SHF_ALLOC sections. I posted a patch that partially fixes this, but it's both the wrong approach and doesn't handle multiple input files with debug info (wrong relocation values). The first issue is representing non SHF_ALLOC atoms in the Atom model. We currently don't have a type for this, and DefinedAtom.cpp makes assumptions about the permissions of an Atom based on their type, so it's hard to use an existing type. The next problem is in the ELF writer. It currently cannot handle AtomSections that are not in a segment as file offsets and addresses are never set. This means that assignOffsets is not called, and that the atoms within are never added to the _atomToAddressMap. However, we can't just add them to that map with their virtual address, as they don't have a virtual address. We need to use the symbol value, which is the offset into the section. My current hack to fix this is to call assignVirtualAddresses(-fileoffset) and then explicitly added them to the _atomToAddressMap. Any ideas for the proper fix here? - Michael Spencer -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130916/956a47ec/attachment.html>
Hi Michael, On 9/16/2013 7:23 PM, Michael Spencer wrote:> Debug info linking is currently broken due to how we handle reading and > laying out non SHF_ALLOC sections. I posted a patch that partially fixes > this, but it's both the wrong approach and doesn't handle multiple input > files with debug info (wrong relocation values). > > The first issue is representing non SHF_ALLOC atoms in the Atom model. We > currently don't have a type for this, and DefinedAtom.cpp makes assumptions > about the permissions of an Atom based on their type, so it's hard to use > an existing type.Can we parse the Debug sections into atoms too ? This way we could associate Debug information associated with DefinedAtoms (seperate reference types, probably). The advantage of this approach would be that Garbage collection would remove all the unneeded references automatically when the definedatom is removed. I think Nick also mentioned about a similiar way a while back.> The next problem is in the ELF writer. It currently cannot handle > AtomSections that are not in a segment as file offsets and addresses are > never set. This means that assignOffsets is not called, and that the atoms > within are never added to the _atomToAddressMap. However, we can't just add > them to that map with their virtual address, as they don't have a virtual > address. We need to use the symbol value, which is the offset into the > section. My current hack to fix this is to call > assignVirtualAddresses(-fileoffset) and then explicitly added them to the > _atomToAddressMap. > > Any ideas for the proper fix here?There is a way that we can handle this without lot of tweaks. a) Assign the debug sections part of a linker internal segment(the segment would not appear in the output file), hasOutputSegment will return true for a debug section. b) Around lines 623, in DefaultLayout, we find out if the section is associated with the special debug section, we add this sepecial segment to the list of segments c) Around lines 731 in DefaultLayout.h, we compare the segment type against the linker internal segment types, and assign offsets for the debug section. Lets not set the virtual addresses for these sections. If there is a need for assigning virtual addresses, you could change the second loop that assigns virtual addresses to deal with that too. Thanks Shankar Easwaran -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation
On Sep 16, 2013, at 6:48 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:> Hi Michael, > On 9/16/2013 7:23 PM, Michael Spencer wrote: >> Debug info linking is currently broken due to how we handle reading and >> laying out non SHF_ALLOC sections. I posted a patch that partially fixes >> this, but it's both the wrong approach and doesn't handle multiple input >> files with debug info (wrong relocation values). >> >> The first issue is representing non SHF_ALLOC atoms in the Atom model. We >> currently don't have a type for this, and DefinedAtom.cpp makes assumptions >> about the permissions of an Atom based on their type, so it's hard to use >> an existing type.There is a couple of ways to model this: 1) SHF_ALLOC=0 sections do not occupy space during execution, so they do not need addresses, so they are not atoms. Instead that section information is modeled as some kind of “attribute” (like a name or content type) of DefinedAtoms or the whole File. 2) If the dwarf can be parsed into chunks that can be associated with DefinedAtoms, then 2a) those chunks could be new attributes of an atom, or 2b) those chunks could be atoms themselves (with some defined way to set the name, permissions, etc of those new atoms) I have looked at breaking up the source line table information in dwarf. Conceptually, it is a table of pc ranges to source file ranges. The problem is that it is a compressed table. Which means you have to decompress it to figure out which rows belong to which atoms. So the big question, is should lld parse dwarf into some internal representation (like it does for sections into atoms), or should it basically just pass-thru and concatenate the dwarf? For what is is worth, Apple has purposefully side stepped this issue with our work flow. The darwin linker always ignores all dwarf debug info in .o files. Instead, it records the code ranges along with the path to .o files into “debug notes” it puts in the linker output file. Our debugger, when it needs debug info for a range, looks at the notes, finds the original .o file and uses the dwarf from it. -Nick> Can we parse the Debug sections into atoms too ? This way we could associate Debug information associated with DefinedAtoms (seperate reference types, probably). > > The advantage of this approach would be that Garbage collection would remove all the unneeded references automatically when the definedatom is removed. > > I think Nick also mentioned about a similiar way a while back.
On Mon, Sep 16, 2013 at 6:48 PM, Shankar Easwaran <shankare at codeaurora.org>wrote:> Hi Michael, > > > On 9/16/2013 7:23 PM, Michael Spencer wrote: > >> Debug info linking is currently broken due to how we handle reading and >> laying out non SHF_ALLOC sections. I posted a patch that partially fixes >> this, but it's both the wrong approach and doesn't handle multiple input >> files with debug info (wrong relocation values). >> >> The first issue is representing non SHF_ALLOC atoms in the Atom model. We >> currently don't have a type for this, and DefinedAtom.cpp makes >> assumptions >> about the permissions of an Atom based on their type, so it's hard to use >> an existing type. >> > Can we parse the Debug sections into atoms too ? This way we could > associate Debug information associated with DefinedAtoms (seperate > reference types, probably). > > The advantage of this approach would be that Garbage collection would > remove all the unneeded references automatically when the definedatom is > removed. > > I think Nick also mentioned about a similiar way a while back.You can't just parse it by byte ranges and pull some out. You would actually need to parse the DWARF and rewrite it.> > > The next problem is in the ELF writer. It currently cannot handle >> AtomSections that are not in a segment as file offsets and addresses are >> never set. This means that assignOffsets is not called, and that the atoms >> within are never added to the _atomToAddressMap. However, we can't just >> add >> them to that map with their virtual address, as they don't have a virtual >> address. We need to use the symbol value, which is the offset into the >> section. My current hack to fix this is to call >> assignVirtualAddresses(-**fileoffset) and then explicitly added them to >> the >> _atomToAddressMap. >> >> Any ideas for the proper fix here? >> > There is a way that we can handle this without lot of tweaks. > > a) Assign the debug sections part of a linker internal segment(the segment > would not appear in the output file), hasOutputSegment will return true for > a debug section. > > b) Around lines 623, in DefaultLayout, we find out if the section is > associated with the special debug section, we add this sepecial segment to > the list of segments >The problem with this is that it's not just debug. We need to properly implement ELF semantics (ok, really gnu-ld semantics and the ELF spec doesn't say what should happen here (or anywhere really)). The semantics seem to be that the value of symbols in non SHF_ALLOC sections is their offset within the section.> > c) Around lines 731 in DefaultLayout.h, we compare the segment type > against the linker internal segment types, and assign offsets for the debug > section. Lets not set the virtual addresses for these sections. If there is > a need for assigning virtual addresses, you could change the second loop > that assigns virtual addresses to deal with that too. > > Thanks > > Shankar Easwaran > > -- > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted > by the Linux Foundation > >- Michael Spencer -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130917/9f5bf6a6/attachment.html>
On Mon, Sep 16, 2013 at 8:23 PM, Michael Spencer <bigcheesegs at gmail.com>wrote:> Debug info linking is currently broken due to how we handle reading and > laying out non SHF_ALLOC sections. I posted a patch that partially fixes > this, but it's both the wrong approach and doesn't handle multiple input > files with debug info (wrong relocation values). > > The first issue is representing non SHF_ALLOC atoms in the Atom model. We > currently don't have a type for this, and DefinedAtom.cpp makes assumptions > about the permissions of an Atom based on their type, so it's hard to use > an existing type. > > The next problem is in the ELF writer. It currently cannot handle > AtomSections that are not in a segment as file offsets and addresses are > never set. This means that assignOffsets is not called, and that the atoms > within are never added to the _atomToAddressMap. However, we can't just add > them to that map with their virtual address, as they don't have a virtual > address. We need to use the symbol value, which is the offset into the > section. My current hack to fix this is to call > assignVirtualAddresses(-fileoffset) and then explicitly added them to the > _atomToAddressMap. > > Any ideas for the proper fix here? >This seems like a problem that will be solved along the way of implementing `ld -r`, so the "proper" solution will probably be something in the general direction of `ld -r`. -- Sean Silva -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130917/3b530490/attachment.html>
On 9/17/2013 3:06 PM, Sean Silva wrote:> This seems like a problem that will be solved along the way of implementing > `ld -r`, so the "proper" solution will probably be something in the general > direction of `ld -r`. > > -- Sean SilvaYou are right Sean. That was the general idea behind the comment that I mentioned in the previous mail, for ld -r, no section would be part of a loadable segment, which will make the current code work. Thanks Shankar Easwaran -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation
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