Messages in this thread | | | Date | Thu, 13 Jun 2019 11:09:47 -0500 | From | Josh Poimboeuf <> | Subject | Re: [RFC V2 00/16] objtool: Add support for Arm64 |
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On Thu, Jun 13, 2019 at 04:55:31PM +0100, Raphael Gault wrote: > Hi Josh, > > On 5/28/19 11:24 PM, Josh Poimboeuf wrote: > > On Tue, May 21, 2019 at 12:50:57PM +0000, Raphael Gault wrote: > > > Hi Josh, > > > > > > Thanks for offering your help and sorry for the late answer. > > > > > > My understanding is that a table of offsets is built by GCC, those > > > offsets being scaled by 4 before adding them to the base label. > > > I believe the offsets are stored in the .rodata section. To find the > > > size of that table, it is needed to find a comparison, which can be > > > optimized out apprently. In that case the end of the array can be found > > > by locating labels pointing to data behind it (which is not 100% safe). > > > > > > On 5/16/19 3:29 PM, Josh Poimboeuf wrote: > > > > On Thu, May 16, 2019 at 11:36:39AM +0100, Raphael Gault wrote: > > > > > Noteworthy points: > > > > > * I still haven't figured out how to detect switch-tables on arm64. I > > > > > have a better understanding of them but still haven't implemented checks > > > > > as it doesn't look trivial at all. > > > > > > > > Switch tables were tricky to get right on x86. If you share an example > > > > (or even just a .o file) I can take a look. Hopefully they're somewhat > > > > similar to x86 switch tables. Otherwise we may want to consider a > > > > different approach (for example maybe a GCC plugin could help annotate > > > > them). > > > > > > > > > > The case which made me realize the issue is the one of > > > arch/arm64/kernel/module.o:apply_relocate_add: > > > > > > ``` > > > What seems to happen in the case of module.o is: > > > 334: 90000015 adrp x21, 0 <do_reloc> > > > which retrieves the location of an offset in the rodata section, and a > > > bit later we do some extra computation with it in order to compute the > > > jump destination: > > > 3e0: 78625aa0 ldrh w0, [x21, w2, uxtw #1] > > > 3e4: 10000061 adr x1, 3f0 <apply_relocate_add+0xf8> > > > 3e8: 8b20a820 add x0, x1, w0, sxth #2 > > > 3ec: d61f0000 br x0 > > > ``` > > > > > > Please keep in mind that the actual offsets might vary. > > > > > > I'm happy to provide more details about what I have identified if you > > > want me to. > > > > I get the feeling this is going to be trickier than x86 switch tables > > (which have already been tricky enough). > > > > On x86, there's a .rela.rodata section which applies relocations to > > .rodata. The presence of those relocations makes it relatively easy to > > differentiate switch tables from other read-only data. For example, we > > can tell that a switch table ends when either a) there's not a text > > relocation or b) another switch table begins. > > > > But with arm64 I don't see a deterministic way to do that, because the > > table offsets are hard-coded in .rodata, with no relocations. > > > > From talking with Kamalesh I got the impression that we might have a > > similar issue for powerpc. > > > > So I'm beginning to think we'll need compiler help. Like a GCC plugin > > that annotates at least the following switch table metadata: > > > > - Branch instruction address > > - Switch table address > > - Switch table entry size > > - Switch table size > > > > The GCC plugin could write all the above metadata into a special section > > which gets discarded at link time. I can look at implementing it, > > though I'll be traveling for two out of the next three weeks so it may > > be a while before I can get to it. > > > > I am completely new to GCC plugins but I had a look and I think I found a > possible solution to retrieve at least part of this information using the > RTL representation in GCC. I can't say it will work for sure but I would be > happy to discuss it with you if you want. > Although there are still some area I need to investigate related to > interacting with the RTL representation and storing info into the ELF > I'd be interested in giving it a try, if you are okay with that.
Sounds promising. I've been stretched thin lately with other work, and I'll be out again next week, so please go ahead :-)
-- Josh
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