Re: RISC-V 32-bit debug builds reaching breaking point: too many symbols

[Abhigyan ghosh <zscript.team.zs@gmail.com>]

Hi Marco,

Thanks for this incredibly detailed analysis — those stats are eye-opening.

I had a quick question as I'm exploring ELF internals and kernel compilation workflows, especially in the RISC-V context:

Would using split debug info earlier in the build (e.g. extracting `.debug_*` before `vmlinux.o` grows too large) help mitigate the symbol table overflow — or is the linker still forced to carry them through regardless due to reloc dependencies?

Also, does this behavior differ between GNU ld and LLD in your tests?

Appreciate the time you've taken to compile all this — the plots and breakdown by section/reloc type really help visualize the scale.

Best regards,  
Abhigyan Ghosh  
@zscript.team.zs (zsml.zscript.org)

On 18 June 2025 4:25:08 pm IST, Marco Bonelli <marco@mebeim.net> wrote:
>RISC-V debug builds generate *millions* of symbols in vmlinux.o, and this number
>is now getting beyond the limit of the maximum symbol table index that can be
>represented by 32-bit Rela relocations. The ELF32_R_SYM portion of Elf32_Rela
>r_info is only 24 bits, therefore symtab indexes larger than 16777215 overflow
>and cause bogus Rela entries pointing to wrong symbols, ultimately breaking the
>build.
>
>I recently noticed this [1] when "MODPOST vmlinux.symvers" failed with thousands
>of errors and warnings on a particular build of mine for v6.15 RISC-V 32-bit.
>
>The majority (99%) of the symbols are for local temporary labels (.Lxxx) that
>would normally be stripped by default at link time, but seems they cannot be
>stripped as they are referenced by Rela relocations. The number of such symbols
>has always been huge from the very first RISC-V Linux version (v4.15) and has
>been steadily growing since (I plotted a bar chart for v6.5-v6.15 here [3]).
>
>For reference, on v4.15 a simple defconfig + debug build with GCC 11.1 produces
>a vmlinux.o with around 7 million such symbols. On v6.15 with GCC 14.2 it gets
>to around 15 million. We are close to the 16.8M limit, and already exceeding it
>on some configurations (the discussion in [1] is an example).
>
>What can be done to reduce them down to an acceptable number, or even better get
>rid of them entirely? These local temporary symbols referenced by relocations
>seem to be ubiquitous, so I suppose this is some RISC-V ELF ABI design choice.
>Could those Rela relocs just avoid referencing any symbol? Or could Rel/Relr be
>used instead?
>
>Otherwise, if those really need to be kept as is, perhaps splitting debug info
>out of vmlinux.o before linking into final vmlinux (when those are finally
>stripped) could also be a viable solution, though the debug info would have to
>be split into multiple files or the same problem would arise.
>
>Thoughts?
>
>Here are some stats from a custom script I hacked together (can provide source
>if needed).
>
>	# Clean v6.15 tree
>	export PATH=/path/to/gcc-14.2.0-nolibc/riscv32-linux/bin:$PATH
>	export ARCH=riscv CROSS_COMPILE=riscv32-linux-
>
>	make defconfig
>	make 32-bit.config
>	./scripts/config -e DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>	make olddefconfig
>	make -j vmlinux
>
>Stats:
>
>	Total symbols: 15138556
>	Temporary local symbols: 14972044
>	Referenced by relocations: 14971527
>	By section:
>		.rela.debug_info                  8783573
>		.rela.debug_line                  3649412
>		.rela.debug_ranges                1110915
>		.rela.debug_loc                   972349
>		.rela.text                        398789
>		.rela.debug_frame                 283422
>		.rela.rodata                      17182
>		.rela.init.text                   15732
>		.rela__bug_table                  11163
>		.rela__jump_table                 10830
>		.rela.debug_aranges               10253
>		.rela.alternative                 4510
>		.rela.data                        3504
>		.rela.text.unlikely               2650
>		.rela__ex_table                   2044
>		.rela.sched.text                  1405
>		.rela.init.rodata                 541
>		.rela.exit.text                   538
>		.rela.ref.text                    344
>		.rela.init.data                   321
>		.rela.noinstr.text                234
>		.rela.spinlock.text               194
>		.rela.data..ro_after_init         98
>		.rela.cpuidle.text                36
>		.rela.srodata                     30
>		.rela__modver                     27
>		.rela.head.text                   21
>		.rela.sdata                       15
>		.rela.irqentry.text               12
>		.relaruntime_shift_d_hash_shift   10
>		.relaruntime_ptr_dentry_hashtable 10
>		.rela.lsm_info.init               3
>		.rela.data..percpu                1
>	By relocation kind:
>		R_RISCV_32           9665623
>		R_RISCV_SUB16        3652043
>		R_RISCV_ADD16        3647245
>		R_RISCV_ADD32        1174187
>		R_RISCV_SUB32        345881
>		R_RISCV_BRANCH       168074
>		R_RISCV_RVC_BRANCH   104031
>		R_RISCV_SET6         87790
>		R_RISCV_SUB6         87790
>		R_RISCV_PCREL_LO12_I 81819
>		R_RISCV_RVC_JUMP     76539
>		R_RISCV_SET8         29260
>		R_RISCV_SUB8         29260
>		R_RISCV_PCREL_HI20   23416
>		R_RISCV_SET16        4798
>		R_RISCV_JAL          4676
>		R_RISCV_PCREL_LO12_S 1410
>		R_RISCV_CALL_PLT     1
>
>[1]: https://lore.kernel.org/lkml/960240908.630790.1748641210849@privateemail.com/
>[2]: https://x.com/mebeim/status/1934950596693635410
>
>--
>Marco Bonelli
>

aghosh