csu: Add missing .cfi_undefined directives #1859
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Without these libunwind attempts to unwind beyond _start which results in nonsense infinite backtraces. Found this by running the libunwind tests for RISCV and noticed that purecap was also missing these directives: ``` starting test1 Frame 1: _Z9backtracei+0x2e Frame 2: _Z5test1i+0x40 Frame 3: main+0x28 Frame 4: __libc_start1+0x76 Frame 5: __start+0x18 Frame 6: _start+0x22 Frame 7: _Z9backtracei+0x2e Frame 8: _start+0x22 Frame 9: _Z9backtracei+0x2e Frame 10: _start+0x22 .... ``` For RISC-V I added the directive to the C function rather than the assembly one but I could also add it to _start instead.
bsdjhb
approved these changes
Sep 22, 2023
| @@ -76,6 +76,7 @@ _start(void *auxv, | |||
| void (*cleanup)(void), /* from shared loader */ | |||
| struct Struct_Obj_Entry *obj) /* from shared loader */ | |||
| { | |||
| __asm__ volatile(".cfi_undefined c30"); | |||
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This has changed upstream in a grosser way by rewriting all of _start in assembly.
It would be really nice if upstream LLVM would support some sort of function attribute to indicate a function that doesn't have a parent frame so that we could annotate these functions with that and LLVM would emit the proper .cfi_undefined directive for the relevant architecture.
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Without these libunwind attempts to unwind beyond _start which results in nonsense infinite backtraces. Found this by running the libunwind tests for RISCV and noticed that purecap was also missing these directives:
For RISC-V I added the directive to the C function rather than the assembly one but I could also add it to _start instead.