On 5/14/26 09:55, Matt Turner wrote:
QEMU used MOVW(2) (0x9300), which loads the syscall number from PC+4,
instead of the kernel's MOVW(7) (0x9305), which loads from PC+14. The
kernel uses five "or r0,r0" nop pads between TRAP_NOARG and the syscall
number word to reach that offset. libunwind's unw_is_signal_frame checks
for the exact kernel byte pattern 0xc3109305 at the frame PC, so QEMU's
compact layout was not detected, breaking unwinding through signal frames.
Expand each trampoline from 6 to 16 bytes matching the kernel layout
defined in arch/sh/kernel/signal_32.c:
#define MOVW(n) (0x9300|((n)-2)) /* Move mem word at PC+n to R3 */
#define TRAP_NOARG 0xc310 /* Syscall w/no args (NR in R3) */
#define OR_R0_R0 0x200b /* or r0,r0 (insert to avoid hardware
bug) */
__put_user(MOVW(7), &frame->retcode[0]); /* 0x9305 */
__put_user(TRAP_NOARG, &frame->retcode[1]); /* 0xc310 */
__put_user(OR_R0_R0, &frame->retcode[2]); /* 0x200b */
__put_user(OR_R0_R0, &frame->retcode[3]); /* 0x200b */
__put_user(OR_R0_R0, &frame->retcode[4]); /* 0x200b */
__put_user(OR_R0_R0, &frame->retcode[5]); /* 0x200b */
__put_user(OR_R0_R0, &frame->retcode[6]); /* 0x200b */
__put_user((__NR_sigreturn), &frame->retcode[7]);
The first two halfwords (MOVW(7) || TRAP_NOARG = 0xc3109305) form the
32-bit value libunwind checks at the frame PC, followed by two
OR_R0_R0 halfwords (0x200b200b) at PC+4. The same layout applies to
the rt_sigreturn trampoline (lines 366-373 of signal_32.c).
Neither this fix nor the companion tuc_link fix is independently
sufficient: this fix makes signal frames detectable but register reads
remain garbage without the correct ucontext layout; that fix corrects the
ucontext layout but libunwind still cannot detect the frame without the
correct trampoline pattern. Together they fix the following libunwind
tests on a 64-bit host:
Gtest-sig-context, Gtest-trace, Ltest-init-local-signal,
Ltest-sig-context, Ltest-trace
Signed-off-by: Matt Turner<[email protected]>
Cc:[email protected]
---
linux-user/sh4/signal.c | 38 ++++++++++++++++++++++++++++++--------
1 file changed, 30 insertions(+), 8 deletions(-)
Reviewed-by: Richard Henderson <[email protected]>
Since you're looking at sh4, any chance you'd have time to create the vdso?
See e.g. linux-user/i386/{Makefile.vdso,vdso.S,vdso.ld}.
r~
