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. -- Josh
