hermiticity issues with ARCH=um

[nickdesaulniers]

via https://lore.kernel.org/lkml/20220921064855.2841607-1-davidgow@google.com/ there's a comment:

Note that this still doesn't seem to be working properly with make
LLVM=1.

I checked our CI and sure enough I can see warnings from /usr/bin/ld.
https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/3095779516/jobs/5012260390
That's unexpected for LLVM=1 builds.

[nathanchance]

This is because UML links the final kernel and vDSO with $(CC), instead of $(LD), which means we use the default linker for the platform by default, which is ld.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh#n78
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/um/vdso/Makefile#n63

We should be able to stick -fuse-ld=lld somewhere in a similar vein as #774, as untangling the $(CC) linking might be a little difficult because UML is linked against user space libraries, so having the driver be the compiler makes sense.

[nickdesaulniers]

because UML is linked against user space libraries, so having the driver be the compiler makes sense.

That shouldn't be a problem; you can pass -### to the compiler to observe precisely how it invokes the linker to see how various compiler flags translate to linker flags.

[sulix]

I tried this with --fuse-ld=lld and am getting a bunch of nasty errors from UML, largely to do with relocations and PIE. (This is not dissimilar to some of the issues we're seeing with Rust, as well: Rust-for-Linux#881).

So it looks like the hermeticity issues with $(LD) are only part of the issue here, and lld is presumably a bit stricter with what it accepts than ld.

Details below:

Patch

diff --git a/arch/um/Makefile b/arch/um/Makefile
index f1d4d67157be..01d9eae736be 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -139,6 +139,9 @@ ifeq ($(CONFIG_LD_IS_BFD),y)
 LDFLAGS_EXECSTACK += $(call ld-option,--no-warn-rwx-segments)
 endif
 
+# Since we're using CC as the driver, we need to force LLD if it is requested.
+LINK-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
+
 LD_FLAGS_CMDLINE = $(foreach opt,$(KBUILD_LDFLAGS),-Wl,$(opt))
 
 # Used by link-vmlinux.sh which has special support for um link

Errors (CONFIG_STATIC_LINK=n)

ERROR:root:ld.lld: error: relocation R_X86_64_64 cannot be used against symbol '__init_begin'; recompile with -fPIC                                                                                                               [585/1935]
>>> defined in ./arch/um/kernel/vmlinux.lds:170                                                                       
>>> referenced by main.c                                                                                                                                                                                                                    
>>>               main.o:(.text+0x2) in archive init/built-in.a                                                                                                                                                                             
                                                                                                                                                                                                                                            
ld.lld: error: relocation R_X86_64_64 cannot be used against symbol '__init_end'; recompile with -fPIC                                                                                                                                      
>>> defined in ./arch/um/kernel/vmlinux.lds:172                                                                                                                                                                                             
>>> referenced by main.c                                                                                                                                                                                                                    
>>>               main.o:(.text+0xC) in archive init/built-in.a                                                       
                                                                                                                                                                                                                                            
ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC                                                                                                                                             
>>> defined in init/built-in.a(main.o)                                                                                
>>> referenced by main.c                                                                                              
>>>               main.o:(.text+0x16) in archive init/built-in.a                                                      
                                                                                                                      
ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(run_init_process) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(run_init_process) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(run_init_process) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(run_init_process) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(run_init_process) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(run_init_process) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(run_init_process) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC                                                                                                                                   [535/1935]
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(warn_bootconfig) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(init_setup) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(init_setup) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(rdinit_setup) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(rdinit_setup) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(parse_early_options) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(parse_early_options) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against symbol '__setup_end'; recompile with -fPIC
>>> referenced by main.c
>>>               main.o:(do_early_param) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against symbol '__setup_start'; recompile with -fPIC
>>> referenced by main.c
>>>               main.o:(do_early_param) in archive init/built-in.a

ld.lld: error: relocation R_X86_64_64 cannot be used against local symbol; recompile with -fPIC
>>> defined in init/built-in.a(main.o)
>>> referenced by main.c
>>>               main.o:(do_early_param) in archive init/built-in.a

ld.lld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Errors (CONFIG_STATIC_LINK=y)

ERROR:root:ld.lld: error: section: .got is not contiguous with other relro sections
ld.lld: error: section: .tdata is not contiguous with other relro sections
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[nathanchance]

With the -no-pie change in #1982 and linking with -Wl,-z,norelro to avoid the error: section: ... is not contiguous with other relro sections error, I can successfully link a kernel.

diff --git a/arch/um/Makefile b/arch/um/Makefile
index 34957dcb88b9..dd42248dead8 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -120,6 +120,9 @@ LINK-$(call gcc-min-version, 60100)$(CONFIG_CC_IS_CLANG) += -no-pie
 endif
 LINK-$(CONFIG_LD_SCRIPT_DYN_RPATH) += -Wl,-rpath,/lib

+# Since we're using CC as the driver, we need to force LLD if it is requested.
+LINK-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld) -Wl,-z,norelro
+
 CFLAGS_NO_HARDENING := $(call cc-option, -fno-PIC,) $(call cc-option, -fno-pic,) \
        -fno-stack-protector $(call cc-option, -fno-stack-protector-all)

Unfortunately, that kernel does not boot for me, at least with clang-17 and clang-18:

$ boot-uml.py -k .
...
<5>Linux version 6.7.0-00001-g3e57d05b0b07-dirty (nathan@dev-arch.thelio-3990X) (ClangBuiltLinux clang version 17.0.6 (https://github.com/llvm/llvm-project 6009708b4367171ccdbf4b5905cb6a803753fe18), ClangBuiltLinux LLD 17.0.6) #1 Wed Jan 17 17:36:03 MST 2024
...
<5>Sorting __ex_table...
<6>Built 1 zonelists, mobility grouping on.  Total pages: 17182
<6>mem auto-init: stack:all(zero), heap alloc:off, heap free:off
<6>Memory: 57636K/69684K available (3949K kernel code, 1098K rwdata, 1192K rodata, 162K init, 225K bss, 12048K reserved, 0K cma-reserved)
<6>SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
<6>NR_IRQS: 64
<6>clocksource: timer: mask: 0xffffffffffffffff max_cycles: 0x1cd42e205, max_idle_ns: 881590404426 ns
<6>Calibrating delay loop... 8293.58 BogoMIPS (lpj=41467904)
<6>Checking that host ptys support output SIGIO...Yes
<6>pid_max: default: 32768 minimum: 301
<4>
<4>Modules linked in:
<6>Pid: 0, comm: swapper Not tainted 6.7.0-00001-g3e57d05b0b07-dirty
<6>RIP: 0033:net_ns_init+0x38/0x1ae
<6>RSP: 000000006053bf30  EFLAGS: 00010202
<6>RAX: 0000000060c02c00 RBX: 0000000060653c9c RCX: 00000000604d2038
<6>RDX: 0000000060c02ba0 RSI: 0000000000000000 RDI: 0000000060567ec0
<6>RBP: 000000006053bf40 R08: 00000000000001bc R09: 0000000000000000
<6>R10: 00000000643046f0 R11: 00000000000001c4 R12: 0000000000000000
<6>R13: 000000006005eb9b R14: 0000000060645f95 R15: 00000000603f095e
<0>Kernel panic - not syncing: Segfault with no mm
<4>CPU: 0 PID: 0 Comm: swapper Not tainted 6.7.0-00001-g3e57d05b0b07-dirty #1
<4>Stack:
<4> 600191f4 60653c9c 6053bf80 600016a1
<4> 60531000 6053bfb0 00000000 00000000
<4> 00000000 ffffffffffffc000 6053bfa0 600040e5
<4>Call Trace:
<4> [<600191f4>] ? net_ns_init+0x0/0x1ae
<4> [<600016a1>] start_kernel+0x3c4/0x4c5
<4> [<600040e5>] start_kernel_proc+0x50/0x58
<4> [<600238b6>] new_thread_handler+0x9a/0xd1
<4> [<60026822>] uml_finishsetup+0x55/0x5a
...