The fsgsbase test's load_gs() helper needs a nonzero-based FS/GS
segment, i.e. a selector whose hidden base differs from the kernel's
saved thread base, to exercise the selector/base state tracking.
64-bit userspace can only create such a segment via modify_ldt() or,
failing that, via the 32-bit set_thread_area syscall over int $0x80.
On kernels built with CONFIG_MODIFY_LDT_SYSCALL=n, modify_ldt() fails
and the test falls back to int $0x80. If the kernel is also built
with CONFIG_IA32_EMULATION=n, no IDT gate is installed for vector
0x80, so executing int $0x80 raises a #GP fault and the test dies
with SIGSEGV instead of reporting results:
traps: fsgsbase_64[5460] general protection fault ip:4012a1
sp:7f38eb124de0 error:402 in fsgsbase_64[12a1,400000+2000]
With both options disabled, pure 64-bit userspace cannot install a
nonzero-based FS/GS segment at all, so this scenario is untestable.
Probe for a working int $0x80 at startup and skip the affected
subtests when neither mechanism is available instead of crashing.
While at it, replace the magic syscall numbers in the int $0x80
inline asm with named __NR_ia32_getpid / __NR_ia32_set_thread_area
constants. These deliberately use the ia32 syscall table numbering,
which differs from the x86-64 numbering exported by <sys/syscall.h>
on 64-bit builds.
Fixes: 0051202f6ad5f ("selftests/x86: Test the FSBASE/GSBASE API and context
switching")
Signed-off-by: Bjoern Doebel <[email protected]>
Assisted-by: opencode:kimi-k3
Cc: [email protected]
---
tools/testing/selftests/x86/fsgsbase.c | 51 +++++++++++++++++++++++++-
1 file changed, 49 insertions(+), 2 deletions(-)
diff --git a/tools/testing/selftests/x86/fsgsbase.c
b/tools/testing/selftests/x86/fsgsbase.c
index 0a75252d31b6a..8538ceb6c5785 100644
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -218,6 +218,40 @@ static void do_remote_base()
static __thread int set_thread_area_entry_number = -1;
+/*
+ * int $0x80 dispatches through the ia32 syscall table, whose numbers
+ * differ from the x86-64 table exposed by <sys/syscall.h> on an
+ * x86_64 build. Define the ia32 numbers we need explicitly.
+ */
+#define __NR_ia32_getpid 20
+#define __NR_ia32_set_thread_area 243
+
+static bool have_int80;
+
+static void sigsegv_int80(int sig, siginfo_t *si, void *ctx_void)
+{
+ siglongjmp(jmpbuf, 1);
+}
+
+static bool probe_int80(void)
+{
+ /*
+ * Check whether int $0x80 is available. Kernels built without
+ * CONFIG_IA32_EMULATION do not install an IDT entry for vector
+ * 0x80, so executing int $0x80 causes a #GP fault.
+ */
+ sethandler(SIGSEGV, sigsegv_int80, 0);
+ if (sigsetjmp(jmpbuf, 1) == 0) {
+ long ret;
+ /* getpid -- harmless if it works */
+ asm volatile ("int $0x80" : "=a" (ret) : "a"
(__NR_ia32_getpid));
+ clearhandler(SIGSEGV);
+ return true;
+ }
+ clearhandler(SIGSEGV);
+ return false;
+}
+
static unsigned short load_gs(void)
{
/*
@@ -245,7 +279,7 @@ static unsigned short load_gs(void)
printf("\tusing LDT slot 0\n");
asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0x7));
return 0x7;
- } else {
+ } else if (have_int80) {
/* No modify_ldt for us (configured out, perhaps) */
struct user_desc *low_desc = mmap(
@@ -260,7 +294,7 @@ static unsigned short load_gs(void)
long ret;
asm volatile ("int $0x80"
: "=a" (ret), "+m" (*low_desc)
- : "a" (243), "b" (low_desc)
+ : "a" (__NR_ia32_set_thread_area), "b" (low_desc)
: "r8", "r9", "r10", "r11");
memcpy(&desc, low_desc, sizeof(desc));
munmap(low_desc, sizeof(desc));
@@ -275,6 +309,9 @@ static unsigned short load_gs(void)
unsigned short gs = (unsigned short)((desc.entry_number << 3) |
0x3);
asm volatile ("mov %0, %%gs" : : "rm" (gs));
return gs;
+ } else {
+ printf("[NOTE]\tno way to create a nonzero-based segment\n");
+ return 0;
}
}
@@ -516,6 +553,11 @@ static void test_ptrace_write_gsbase(void)
gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
+ if (*shared_scratch == 0) {
+ printf("[SKIP]\tCould not create a nonzero GS
selector\n");
+ goto END;
+ }
+
if (gs != *shared_scratch) {
nerrs++;
printf("[FAIL]\tGS is not prepared with nonzero\n");
@@ -587,6 +629,11 @@ int main()
}
clearhandler(SIGILL);
+ /* Probe int $0x80 (32-bit syscall entry) */
+ have_int80 = probe_int80();
+ if (!have_int80)
+ printf("\tint $0x80 is unavailable
(CONFIG_IA32_EMULATION=n?)\n");
+
sethandler(SIGSEGV, sigsegv, 0);
check_gs_value(0);
--
2.50.1
