On 5/30/2025 4:07 PM, Xin Li (Intel) wrote: > When FRED is enabled, if the Trap Flag (TF) is set without an external > debugger attached, it can lead to an infinite loop in the SIGTRAP > handler. To avoid this, the software event flag in the augmented SS > must be cleared, ensuring that no single-step trap remains pending when > ERETU completes. > > This test checks for that specific scenario—verifying whether the kernel > correctly prevents an infinite SIGTRAP loop in this edge case. >
It isn't clear from the commit message whether the test is specific to FRED or a generic one. > Signed-off-by: Xin Li (Intel) <[email protected]> > --- > tools/testing/selftests/x86/Makefile | 2 +- > .../selftests/x86/test_sigtrap_handler.c | 80 +++++++++++++++++++ > 2 files changed, 81 insertions(+), 1 deletion(-) > create mode 100644 tools/testing/selftests/x86/test_sigtrap_handler.c > > diff --git a/tools/testing/selftests/x86/Makefile > b/tools/testing/selftests/x86/Makefile > index f703fcfe9f7c..c486fd88ebb1 100644 > --- a/tools/testing/selftests/x86/Makefile > +++ b/tools/testing/selftests/x86/Makefile > @@ -12,7 +12,7 @@ CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh "$(CC)" > trivial_program.c -no-pie) > > TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt > test_mremap_vdso \ > check_initial_reg_state sigreturn iopl ioperm \ > - test_vsyscall mov_ss_trap \ > + test_vsyscall mov_ss_trap test_sigtrap_handler \ > syscall_arg_fault fsgsbase_restore sigaltstack > TARGETS_C_BOTHBITS += nx_stack > TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \ > diff --git a/tools/testing/selftests/x86/test_sigtrap_handler.c > b/tools/testing/selftests/x86/test_sigtrap_handler.c > new file mode 100644 > index 000000000000..9c5c2cf0cf88 > --- /dev/null > +++ b/tools/testing/selftests/x86/test_sigtrap_handler.c > @@ -0,0 +1,80 @@ > +// SPDX-License-Identifier: GPL-2.0-or-later Curious about your use of GPL-2.0-or-later? All the files in this directory use GPL-2.0-only or GPL-2.0. > +/* > + * Copyright (C) 2025 Intel Corporation > + */ > +#define _GNU_SOURCE > + > +#include <err.h> > +#include <signal.h> > +#include <stdio.h> > +#include <stdlib.h> > +#include <string.h> > +#include <sys/ucontext.h> > + > +#ifdef __x86_64__ > +# define REG_IP REG_RIP > +#else > +# define REG_IP REG_EIP > +#endif > + > +static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *), > int flags) > +{ > + struct sigaction sa; > + > + memset(&sa, 0, sizeof(sa)); > + sa.sa_sigaction = handler; > + sa.sa_flags = SA_SIGINFO | flags; > + sigemptyset(&sa.sa_mask); > + > + if (sigaction(sig, &sa, 0)) > + err(1, "sigaction"); > + > + return; > +} > + > +static unsigned int loop_count_on_same_ip; > + > +static void sigtrap(int sig, siginfo_t *info, void *ctx_void) > +{ > + ucontext_t *ctx = (ucontext_t *)ctx_void; > + static unsigned long last_trap_ip; > + > + if (last_trap_ip == ctx->uc_mcontext.gregs[REG_IP]) { > + printf("trapped on %016lx\n", last_trap_ip); > + > + if (++loop_count_on_same_ip > 10) { > + printf("trap loop detected, test failed\n"); > + exit(2); > + } Most of the x86 selftests use the ksft_exit_fail_msg(), ksft_print_msg() or [RUN, FAIL, OK] style for error messages and other informational prints. > + > + return; > + } > + > + loop_count_on_same_ip = 0; > + last_trap_ip = ctx->uc_mcontext.gregs[REG_IP]; > + printf("trapped on %016lx\n", last_trap_ip); > +} > + > +int main(int argc, char *argv[]) > +{ > + sethandler(SIGTRAP, sigtrap, 0); > + I would suggest a comment here to explain what the following assembly code is supposed to do. It isn't obvious from a cursory look. > + asm volatile( > +#ifdef __x86_64__ > + /* Avoid clobbering the redzone */ > + "sub $128, %rsp\n\t" > +#endif > + "push $0x302\n\t" > + "popf\n\t" > + "nop\n\t" > + "nop\n\t" > + "push $0x202\n\t" > + "popf\n\t" > +#ifdef __x86_64__ > + "add $128, %rsp\n\t" > +#endif > + ); > + > + printf("test passed\n"); > + return 0; > +} > > base-commit: 485d11d84a2452ac16466cc7ae041c93d38929bc
