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Message-ID: <20260708123146.667953-1-alex@alexrp.com>
Date: Wed, 8 Jul 2026 14:31:46 +0200
From: Alex Rønne Petersen <alex@...xrp.com>
To: musl@...ts.openwall.com
Cc: Alex Rønne Petersen <alex@...xrp.com>
Subject: [PATCH] riscv: clear fp and ra in _start like on other ports
As usual, clearing fp helps simple/generic frame-pointer-based unwinders.
Clearing ra unbreaks DWARF unwinders because ra would otherwise contain garbage
from _start all the way through to libc_start_main_stage2. The CFI for
libc_start_main_stage2 would then incorrectly claim that ra contains a perfectly
valid return address that the unwinder should proceed through. If that return
address just so happened to land inside a real function somewhere, the unwinder
would then happily proceed to run that function's CFI on a completely unrelated
register context, unsurprisingly leading to weird crashes.
As an aside: Some ports don't clear their ra equivalent in _start, but they do
use a linked branch to _start_c. That works out too because the unwinder then
actually reaches _start which intentionally has no CFI, so it stops there.
Between these approaches, clearing ra and using an unlinked branch tends to lead
to slightly nicer stack traces for users because there won't be an unwind error
printed for the _start frame (due to missing CFI).
---
arch/riscv32/crt_arch.h | 2 ++
arch/riscv64/crt_arch.h | 2 ++
2 files changed, 4 insertions(+)
diff --git a/arch/riscv32/crt_arch.h b/arch/riscv32/crt_arch.h
index 6b93fcfd..ad556137 100644
--- a/arch/riscv32/crt_arch.h
+++ b/arch/riscv32/crt_arch.h
@@ -10,6 +10,8 @@ START ":\n"
".option norelax\n\t"
"lla gp, __global_pointer$\n"
".option pop\n\t"
+"li fp, 0\n"
+"li ra, 0\n"
"mv a0, sp\n"
".weak _DYNAMIC\n"
".hidden _DYNAMIC\n\t"
diff --git a/arch/riscv64/crt_arch.h b/arch/riscv64/crt_arch.h
index 6b93fcfd..ad556137 100644
--- a/arch/riscv64/crt_arch.h
+++ b/arch/riscv64/crt_arch.h
@@ -10,6 +10,8 @@ START ":\n"
".option norelax\n\t"
"lla gp, __global_pointer$\n"
".option pop\n\t"
+"li fp, 0\n"
+"li ra, 0\n"
"mv a0, sp\n"
".weak _DYNAMIC\n"
".hidden _DYNAMIC\n\t"
--
2.53.0
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