Fund: DynamicLoader+LibELF: Executes unaligned memory accesses on aarch64

We currently build all of userspace and kernel with the -mstrict-align flag to enforce the compiler to emit aligned memory accesses:

serenity/Meta/CMake/serenity_compile_options.cmake

Lines 56 to 61 in ba5df46

    
           if ("${SERENITY_ARCH}" STREQUAL "aarch64") 
        
               # Unaligned memory access will cause a trap, so to make sure the compiler doesn't generate 
        
               # those unaligned accesses, the strict-align flag is added. 
        
               # FIXME: Remove -Wno-cast-align when we are able to build everything without this warning turned on. 
        
               add_compile_options(-mstrict-align -Wno-cast-align) 
        
           endif()

We do this because we set up the aarch64 CPU to enforce aligned memory accesses:

serenity/Kernel/Arch/aarch64/Exceptions.cpp

Line 77 in ba5df46

system_control_register_el1.A = 1; // Enable memory access alignment check

However, the compiler still emits code in the DynamicLoader (or to be more specific in LibELF/Validation.cpp) that will do unaligned memory accesses:

Backtrace (exception happens in ELF::validate_program_headers(Elf64_Ehdr const&, unsigned long, AK::Span<unsigned char const>, AK::StringBuilder*, bool):

void AK::Variant<bool, AK::Error>::set<bool, bool>(bool&&, AK::Detail::VariantNoClearTag) at ./Build/aarch64/././AK/Variant.h:376
ELF::Image::parse() at ./Build/aarch64/./Userland/Libraries/LibELF/Image.cpp:131
ELF::DynamicLoader::DynamicLoader(int, AK::DeprecatedString, void*, unsigned long) at ./Build/aarch64/./Userland/Libraries/LibELF/DynamicLoader.cpp:71
ELF::DynamicLoader::try_create(int, AK::DeprecatedString) at ./Build/aarch64/./Userland/Libraries/LibELF/DynamicLoader.cpp:59
ELF::map_library(AK::DeprecatedString const&, int) at ./Build/aarch64/./Userland/Libraries/LibELF/DynamicLinker.cpp:94
ELF::DynamicLinker::linker_main(AK::DeprecatedString&&, int, bool, int, char**, char**) at ./Build/aarch64/./Userland/Libraries/LibELF/DynamicLinker.cpp:642
_entry at ./Build/aarch64/./Userland/DynamicLoader/main.cpp:112
_start at ./Build/aarch64/./Userland/DynamicLoader/main.cpp:66

Output

➜  aarch64 git:(aarch64-alignment-exception) ✗ ninja Kernel && ../../Meta/run.sh
[0/2] Re-checking globbed directories...
[1/2] Re-running CMake...
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/timon/serenity/Build/aarch64
[0/2] Re-checking globbed directories...
[0/6] cd /Users/timon/serenity/Build/aarch64 && /opt/homebrew/Cellar/cmake/3....LCHAIN=GNU LLVM_VERSION=12.2.0 /Users/timon/serenity/Meta/build-image-qemu.sh
Password:
checking existing image
e2fsck 1.46.5 (30-Dec-2021)
Stap 1: Controle van inodes, blokken, en groottes
Stap 2: Controle van mappenstructuur
Stap 3: Controle van verbindingen tussen mappen
Stap 4: Controle van verwijzingsaantallen
Stap 5: Controle van groepssamenvattingen
_disk_image: 5024/6240 bestanden (0.4% niet-aaneengesloten), 110581/488272 blokken
done
mounting filesystem... done
installing base system... done
creating initial filesystem structure... done
creating utmp file... done
setting up device nodes folder... done
setting up sysfs folder... done
installing users... done
adding some desktop icons... done
installing shortcuts... done
installing 'checksum' variants... done
unmounting filesystem... done
[6/6] Linking CXX executable Kernel/Kernel
/Users/timon/serenity/Toolchain/Local/aarch64/lib/gcc/aarch64-pc-serenity/12.2.0/../../../../aarch64-pc-serenity/bin/ld: warning: -z relro ignored
/Users/timon/serenity/Toolchain/Local/aarch64/lib/gcc/aarch64-pc-serenity/12.2.0/../../../../aarch64-pc-serenity/bin/ld: warning: -z pack-relative-relocs ignored
[Kernel]: Welcome to Serenity OS!
[Kernel]: Imagine this being your ideal operating system.
[Kernel]: Observed deviations from that ideal are shortcomings of your imagination.
[Kernel]:
[Kernel]: CPU[0]: Supports PMULL DoubleLock SHA1 SHA256 CRC32 BBM PMUv3
[Kernel]: CPU[0]: Physical address bit width: 40
[Kernel]: CPU[0]: Virtual address bit width: 48
[Kernel]: CPU[0]: Random number generator not detected, randomness will be poor
[Kernel]: Loading kernel symbol table...
[Kernel]: Kernel Commandline:
[Kernel]: Starting SerenityOS...
[Kernel]: Initialize MMU
[Kernel]: MM: Multiboot mmap: address=0x0000000000000000, length=1056964608, type=1
[Kernel]: MM: boot_pml4t @ P00000000024bf000
[Kernel]: MM: boot_pdpt @ P00000000024c0000
[Kernel]: MM: boot_pd0 @ P0000000000000000
[Kernel]: MM: boot_pd_kernel @ P00000000024e1000
[Kernel]: MM: Physical page entries: 0x0000002002e00000 - 0x0000002002ff8fff (size 0x00000000001f9000)
[Kernel]: MM: Kernel range @ P0000000000080000 - P0000000002cbefff (size 0x2c3f000)
[Kernel]: MM: Boot module range @ P0000000002cbf000 - P00000000209955ff (size 0x1dcd6600)
[Kernel]: MM: Physical Pages range @ P0000000020996000 - P0000000020b8ffff (size 0x1fa000)
[Kernel]: MM: User physical region: P0000000000000000 - P000000000007efff (size 0x7f000)
[Kernel]: MM: User physical region: P0000000020b90000 - P000000003effffff (size 0x1e470000)
[Kernel]:  * 30x PhysicalZone (16 MiB) @ 0000000020b90000-000000003db8ffff
[Kernel]:  * 4x PhysicalZone (1 MiB) @ 000000003eb90000-000000003ee8ffff
[Kernel]: Initialized framebuffer: 1280 x 720 @ 32 bits
[Kernel]: PAT is not supported, implement MTRR fallback if available
[Kernel]: Boot logo size: 274351 (205 x 446)
0.500 [Kernel]: KernelRng: Using bad entropy source TimeManagement
0.551 [#0 colonel(0:0)]: Scheduler[0]: idle loop running
0.557 [init_stage2(1:1)]: Firmware version: 4294967295
0.563 [init_stage2(1:1)]: Ramdisk: Device #0 @ V0x0000002007e30000, Capacity=500002816
0.830 [#0 init_stage2(1:1)]: Master Boot Record: invalid signature
0.830 [#0 init_stage2(1:1)]: Master Boot Record: invalid signature
0.881 [init_stage2(1:1)]: VirtualFileSystem: mounted root(1) from Ext2FS (device:6,0)
1.040 [#0 SystemServer(2:2)]: Unknown DFSC: Alignment fault
1.040 [SystemServer(2:2)]: CRASH: CPU #0 Unknown page fault in userspace
1.040 [#0 SystemServer(2:2)]: Exception Syndrome: EC(0b100100) IL(0b1) ISS(0b1100001) ISS2(0b0)
1.040 [#0 SystemServer(2:2)]:     Class: Data Abort exception from a lower Exception level
1.045 [#0 SystemServer(2:2)]:     Data Fault Status Code: Alignment fault
1.045 [#0 SystemServer(2:2)]:     Faulting Virtual Address: 0x945c7a449
1.045 [#0 SystemServer(2:2)]: Saved Program Status: (NZCV(0b110) DAIF(0b0) M(0b0)) / 0x60000000
1.045 [#0 SystemServer(2:2)]: Exception Link Register: 0x12631174
1.045 [#0 SystemServer(2:2)]: Stack Pointer (EL0): 0x945c7a260
1.045 [#0 SystemServer(2:2)]:  x0=0x0000000945c7a449  x1=0x0000000000000001  x2=0x00000000000001c8  x3=0x0000000000000008  x4=0x000000006474e550
1.049 [#0 SystemServer(2:2)]:  x5=0x000000006474e552  x6=0x000000006474e551  x7=0x000000006474e551  x8=0x0000000000000003  x9=0x0000000000000012
1.049 [#0 SystemServer(2:2)]: x10=0x0000000000012890 x11=0x0000000945c7a340 x12=0x00000000126b7f40 x13=0x0000000000000040 x14=0x0000000000000040
1.049 [#0 SystemServer(2:2)]: x15=0xffffffffffffffc0 x16=0x0000000000000000 x17=0x0000000000000000 x18=0x0000000000000000 x19=0x0000000000130000
1.049 [#0 SystemServer(2:2)]: x20=0x0000000945c7a448 x21=0x00000000126c2000 x22=0x0000000000130000 x23=0x0000000000063540 x24=0x0000000000000040
1.055 [#0 SystemServer(2:2)]: x25=0x0000000000000008 x26=0x0000000000063540 x27=0x0000000000000000 x28=0x0000000000000001 x29=0x0000000945c7a260
1.055 [#0 SystemServer(2:2)]: x30=0x000000001262eb60
1.055 [#0 SystemServer(2:2)]: 0x0000000012631174  (?)

1.055 [#0 SystemServer(2:2)]: Userspace backtrace:
1.055 [#0 SystemServer(2:2)]: 0x000000001262eb60 (next: 0x0000000945c7a3c0)
1.055 [#0 SystemServer(2:2)]: 0x0000000012629f5c (next: 0x0000000945c7a470)
1.055 [#0 SystemServer(2:2)]: 0x000000001262a274 (next: 0x0000000945c7a500)
1.060 [#0 SystemServer(2:2)]: 0x00000000126226a8 (next: 0x0000000945c7a640)
1.060 [#0 SystemServer(2:2)]: 0x0000000012624834 (next: 0x0000000945c7a6f0)
1.060 [#0 SystemServer(2:2)]: 0x00000000125e63dc (next: 0x0000000945c7a900)
1.060 [#0 SystemServer(2:2)]: 0x00000000125e6224 (next: 0x0000000000000000)
1.060 [#0 SystemServer(2:2)]: Kernel backtrace:
1.060 [#0 SystemServer(2:2)]: Kernel + 0x0000000000a1a57c  Kernel::Process::crash(int, AK::Optional<Kernel::RegisterState const&>, bool) +0x2ac
1.060 [#0 SystemServer(2:2)]: Kernel + 0x00000000001756c8  Kernel::handle_crash(Kernel::RegisterState const&, char const*, int, bool) +0x568
1.060 [#0 SystemServer(2:2)]: Kernel + 0x0000000000de3c18  exception_common +0x2a8
1.066 [#0 SystemServer(2:2)]: Kernel + 0x0000000000dfa6bc  synchronous_lower_el +0x74
1.066 [#0 SystemServer(2:2)]: Process regions:
1.066 [#0 SystemServer(2:2)]: BEGIN                 END                SIZE               ACCESS NAME
1.066 [#0 SystemServer(2:2)]: 0x0000000000110000 -- 0x000000000011ffff 0x0000000000010000 RW     malloc: ChunkedBlock(64)
1.066 [#0 SystemServer(2:2)]: 0x0000000000120000 -- 0x000000000012ffff 0x0000000000010000 RW     malloc: ChunkedBlock(32)
1.066 [#0 SystemServer(2:2)]: 0x0000000000130000 -- 0x0000000000193fff 0x0000000000064000 R  S   ELF_DYN: /bin/SystemServer
1.066 [#0 SystemServer(2:2)]: 0x00000000001a0000 -- 0x00000000001affff 0x0000000000010000 RW     malloc: ChunkedBlock(256)
1.066 [#0 SystemServer(2:2)]: 0x00000000125d7000 -- 0x00000000125e0fff 0x000000000000a000 R  S C /usr/lib/Loader.so
1.066 [#0 SystemServer(2:2)]: 0x00000000125e1000 -- 0x0000000012672fff 0x0000000000092000 R XS C /usr/lib/Loader.so
1.066 [#0 SystemServer(2:2)]: 0x0000000012673000 -- 0x00000000126bcfff 0x000000000004a000 R  S C /usr/lib/Loader.so
1.066 [#0 SystemServer(2:2)]: 0x00000000126bd000 -- 0x00000000126c4fff 0x0000000000008000 RW     /usr/lib/Loader.so (data-rw)
1.066 [#0 SystemServer(2:2)]: 0x0000000945b7b000 -- 0x0000000945c7afff 0x0000000000100000 RW  T  Stack (Main thread)
1.066 [#0 SystemServer(2:2)]: 0x00000011282b7000 -- 0x00000011282b7fff 0x0000000000001000 R XS C Signal trampoline
1.066 [#0 SystemServer(2:2)]: Kernel regions:
1.066 [#0 SystemServer(2:2)]: BEGIN                 END                SIZE               ACCESS NAME
1.066 [#0 SystemServer(2:2)]: 0x0000002000000000 -- 0x0000002002dfffff 0x0000000002e00000
1.066 [#0 SystemServer(2:2)]: 0x0000002002e00000 -- 0x0000002002ff8fff 0x00000000001f9000
1.066 [#0 SystemServer(2:2)]: 0x0000002002ff9000 -- 0x0000002002ff9fff 0x0000000000001000 RW     Time page
1.066 [#0 SystemServer(2:2)]: 0x0000002002ffa000 -- 0x0000002002ffafff 0x0000000000001000 R    C Signal trampolines
1.066 [#0 SystemServer(2:2)]: 0x0000002002ffb000 -- 0x0000002002ffbfff 0x0000000000001000 RW     Ext2FS: Block group descriptors
1.066 [#0 SystemServer(2:2)]: 0x0000002002ffc000 -- 0x0000002002ffcfff 0x0000000000001000 RW     Ext2FS: Cached bitmap block
1.066 [#0 SystemServer(2:2)]: 0x0000002002ffd000 -- 0x0000002002ffdfff 0x0000000000001000 RW     Ext2FS: Cached bitmap block
1.066 [#0 SystemServer(2:2)]: 0x0000002002ffe000 -- 0x0000002002ffefff 0x0000000000001000 RW     Ext2FS: Cached bitmap block
1.066 [#0 SystemServer(2:2)]: 0x0000002003000000 -- 0x0000002006ffffff 0x0000000004000000
1.066 [#0 SystemServer(2:2)]: 0x0000002007000000 -- 0x0000002007e0ffff 0x0000000000e10000 RW     Boot Framebuffer
1.066 [#0 SystemServer(2:2)]: 0x0000002007e10000 -- 0x0000002007e1ffff 0x0000000000010000 RW  T  Kernel stack (thread 0)
1.066 [#0 SystemServer(2:2)]: 0x0000002007e20000 -- 0x0000002007e2ffff 0x0000000000010000 RW  T  Kernel stack (thread 1)
1.066 [#0 SystemServer(2:2)]: 0x0000002007e30000 -- 0x0000002025b06fff 0x000000001dcd7000 RW     Ramdisk
1.066 [#0 SystemServer(2:2)]: 0x0000002025b07000 -- 0x00000020264cafff 0x00000000009c4000 RW     BlockBasedFS: Cache blocks
1.066 [#0 SystemServer(2:2)]: 0x00000020264cb000 -- 0x0000002026540fff 0x0000000000076000 RW     BlockBasedFS: Cache entries
1.066 [#0 SystemServer(2:2)]: 0x0000002026541000 -- 0x0000002026550fff 0x0000000000010000 RW  T  Kernel stack (thread 2)
1.066 [#0 SystemServer(2:2)]: 0x0000002026551000 -- 0x0000002026551fff 0x0000000000001000 RW     Ext2FS: Cached bitmap block

SerenityOS/serenity

DynamicLoader+LibELF: Executes unaligned memory accesses on aarch64

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	if ("${SERENITY_ARCH}" STREQUAL "aarch64")
	# Unaligned memory access will cause a trap, so to make sure the compiler doesn't generate
	# those unaligned accesses, the strict-align flag is added.
	# FIXME: Remove -Wno-cast-align when we are able to build everything without this warning turned on.
	add_compile_options(-mstrict-align -Wno-cast-align)
	endif()

SerenityOS/serenity

DynamicLoader+LibELF: Executes unaligned memory accesses on aarch64

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