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arch/arm64: Report a recovered user fault as a segmentation fault. - #20510
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nxstyle reports "Bad right brace alignment" for the closing brace of the switch in arm64_el1_exception_handler(). Indent it like the switch, because CI checks every file that a change touches. No functional change. The change is whitespace only. Assisted-by: Claude Code:claude-opus-5-5 Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
Two problems in the same path make a contained user fault look like a kernel failure. arm64_el1_undef() dumps the words around ELR. For an exception taken from EL0, ELR is a user address, and the words around it can be in a page that is not mapped. Then the memcpy faults inside the fatal handler. That nested exception trips the DEBUGASSERT in arm64_fatal_handler(), and the fault that the user took is not reported. The ESR that tells what happened is lost. Skip the dump when the exception came from EL0. At EL1 the address is kernel code that was just fetched, so keep the dump there. arm64_fatal_handler() then reports the fault that it recovers from as "PANIC: Unhandled user exception", followed by a full register dump. But there is no panic: it sets TCB_FLAG_FORCED_CANCEL, changes ELR to _exit(SIGSEGV), and the system continues without the offending task. Print "Segmentation fault in <process> (PID n: <thread>)" instead, the same message as risc-v, and keep the register dump for the PANIC_WITH_REGS() path, which is fatal. Tested on QEMU qemu-armv8a:knsh with examples/sandbox and ostest. A user read or write of kernel memory (0x40000000) now prints: arm64_exception_handler: ESR_ELn: 0x9200000e arm64_fatal_handler: Segmentation fault in sandbox (PID 10: sandbox) arm64_fatal_handler: Reason: DABT (lower EL) - Data Abort from a ... sandbox: the offender exited with status 2816 Before this change it printed "PANIC: Unhandled user exception" and a register dump for the same recovered fault. An undefined instruction at EL0 now prints "Undefined instruction at <ELR>" without the dump, then the same segmentation fault message. The shell survives in all cases, and ostest exits with status 0 before and after this change. Assisted-by: Claude Code:claude-opus-5-5 Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
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xiaoxiang781216
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Oct 8, 2026
acassis
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Oct 8, 2026
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Summary
On arm64, a user fault that the kernel recovers from looks like a kernel failure.
arm64_fatal_handler()prints "PANIC: Unhandled user exception" and a full register dump, but it then only kills the offending task with SIGSEGV. This change prints "Segmentation fault in (PID n: )", the same message as risc-v, and keeps the register dump for the fatal path.arm64_el1_undef()also reads the words around ELR. For an exception from EL0 these are user addresses that can be in an unmapped page. A fault there is a nested exception that trips the DEBUGASSERT inarm64_fatal_handler(). This change skips the dump for EL0 and keeps it for EL1.The first commit fixes an older nxstyle error in
arm64_fatal.c, because CI checks every file that a change touches. It changes whitespace only.Impact
Only the console output changes, for faults from EL0, which happen in kernel builds. Recovery behaviour does not change. Kernel faults still panic with the register dump.
Testing
QEMU,
qemu-armv8a:knshwith appsexamples/sandboxandostest, Arm GNU Toolchain 14.2.rel1 (aarch64-none-elf).Before (master),
sandbox 0x40000000:After:
An undefined instruction at EL0 (a local test probe, not part of this PR) now prints the following, and the shell survives:
On today's master I could not make the dump read an unmapped page: the word before the start of the text was mapped. The skip for EL0 prevents that case.
ostestexits with status 0 before and after this change.tools/checkpatch.sh -c -u -m -gpasses.