Quantifying the benefits of DT_AARCH64_AUTH_RELR about abi-aa HOT 5 OPEN

Are there figures on how many regular relative relocations are transformed into R_AARCH64_AUTH_RELATIVE under PAuthABI?

The 32b limit was chosen on the basis that in the vast majority of cases (small code model, position independent code) the addend is going to be within the 0..2^32 range. So I would say that in a typical userspace, pretty much every relocation is going to use this.

from abi-aa.

I agree that in the majority of cases linkers don't need to move a relocation from .relr.auth.dyn to .rela.dyn.
The question is do we need this RELR variant?

If we use CREL (while size(.crel.dyn)/size(.relr.dyn) may be 3.5x, size(.crel.dyn)/size(.o) is just 0.3%), supporting AUTH relative will take very few lines in https://github.com/MaskRay/llvm-project/blob/demo-crel/lld/ELF/SyntheticSections.cpp around CrelSection<uint>::updateAllocSize, and the code will be centralized in one place.

from abi-aa.

Got it, I misread your earlier message and thought you were asking about .rela.dyn -> .relr.auth.dyn rather than R_AARCH64_ABS64 -> R_AARCH64_AUTH_ABS64. Let me see if I can collect those numbers for AOSP.

from abi-aa.

I built AOSP with PAuth ABI disabled and enabled for the Cuttlefish (i.e. emulator) arm64 target and ran the following commands from the symbols directory of the build directory, which contains unstripped versions of all built binaries shipped on the device.

$ llvm-readelf -rW (find -type f) | grep 000000000000e20| wc -l
524144
$ llvm-readelf -rW (find -type f) | grep R_AARCH64_RELATIVE| wc -l
725136

So around 40% of R_AARCH64_RELATIVE relocations become R_AARCH64_AUTH_RELATIVE.

Also, the total size of all SHT_RELR sections in a tree of binaries built with PAuth ABI disabled is 460824 bytes and with PAuth ABI enabled is 307032 bytes. So we can estimate that the total DT_AARCH64_AUTH_RELR size if it were implemented would be around 150KB. Assuming a 3.5x ratio for CREL vs RELR that would mean it would cost 375KB in the shipped image not to have DT_AARCH64_AUTH_RELR (i.e. 0.04% of the total image size of 1.1GB), but I would expect the actual ratio to be higher than for DT_RELR since each relocation contains more information.

from abi-aa.