selftests/x86: Fix error: variably modified 'altstack_data' at file scope

A glibc 2.34 feature adds support for variable MINSIGSTKSZ and SIGSTKSZ.
When _DYNAMIC_STACK_SIZE_SOURCE or _GNU_SOURCE are defined, MINSIGSTKSZ
and SIGSTKSZ are no longer constant on Linux. glibc 2.34 flags code paths
assuming MINSIGSTKSZ or SIGSTKSZ are constant. Fix these error in x86 test.

Feature description and build error:

NEWS for version 2.34
=====================
Major new features:
 * Add _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.  When _DYNAMIC_STACK_SIZE_SOURCE
   or _GNU_SOURCE are defined, MINSIGSTKSZ and SIGSTKSZ are no longer
   constant on Linux.  MINSIGSTKSZ is redefined to sysconf(_SC_MINSIGSTKSZ)
   and SIGSTKSZ is redefined to sysconf (_SC_SIGSTKSZ).  This supports
   dynamic sized register sets for modern architectural features like
   Arm SVE.
=====================

If _SC_SIGSTKSZ_SOURCE or _GNU_SOURCE are defined, MINSIGSTKSZ and SIGSTKSZ
are redefined as:

/* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
 # undef SIGSTKSZ
 # define SIGSTKSZ sysconf (_SC_SIGSTKSZ)

/* Minimum stack size for a signal handler: SIGSTKSZ.  */
 # undef MINSIGSTKSZ
 # define MINSIGSTKSZ SIGSTKSZ

Compilation will fail if the source assumes constant MINSIGSTKSZ or
SIGSTKSZ.

Build error with the GNU C Library 2.34:
DEBUG:	| sigreturn.c:150:13: error: variably modified 'altstack_data' at file scope
| sigreturn.c:150:13: error: variably modified 'altstack_data' at file scope
DEBUG:	|   150 | static char altstack_data[SIGSTKSZ];
|   150 | static char altstack_data[SIGSTKSZ];
DEBUG:	|       |             ^~~~~~~~~~~~~

DEBUG:  | single_step_syscall.c:60:22: error: variably modified 'altstack_data' at file scope
DEBUG:  |   60 | static unsigned char altstack_data[SIGSTKSZ];
DEBUG:  |      |                      ^~~~~~~~~~~~~

Fixed commit log to improve formatting and clarity:
Shuah Khan <skhan@linuxfoundation.org>

Link: https://sourceware.org/pipermail/libc-alpha/2021-January/121996.html
Link: https://sourceware.org/pipermail/libc-alpha/2021-August/129718.htmlSuggested-by: Jianwei Hu <jianwei.hu@windriver.com>
Signed-off-by: Jun Miao <jun.miao@windriver.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/x86/mov_ss_trap.c

@@ -47,7 +47,6 @@
 unsigned short ss;
 extern unsigned char breakpoint_insn[];
 sigjmp_buf jmpbuf;
-static unsigned char altstack_data[SIGSTKSZ];
 
 static void enable_watchpoint(void)
 {
@@ -250,13 +249,14 @@ int main()
 	if (sigsetjmp(jmpbuf, 1) == 0) {
 		printf("[RUN]\tMOV SS; SYSENTER\n");
 		stack_t stack = {
-			.ss_sp = altstack_data,
+			.ss_sp = malloc(sizeof(char) * SIGSTKSZ),
 			.ss_size = SIGSTKSZ,
 		};
 		if (sigaltstack(&stack, NULL) != 0)
 			err(1, "sigaltstack");
 		sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND | SA_ONSTACK);
 		nr = SYS_getpid;
+		free(stack.ss_sp);
 		/* Clear EBP first to make sure we segfault cleanly. */
 		asm volatile ("xorl %%ebp, %%ebp; mov %[ss], %%ss; SYSENTER" : "+a" (nr)
 			      : [ss] "m" (ss) : "flags", "rcx"

tools/testing/selftests/x86/sigreturn.c

@@ -138,9 +138,6 @@ static unsigned short LDT3(int idx)
 	return (idx << 3) | 7;
 }
 
-/* Our sigaltstack scratch space. */
-static char altstack_data[SIGSTKSZ];
-
 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
 		       int flags)
 {
@@ -771,7 +768,8 @@ int main()
 	setup_ldt();
 
 	stack_t stack = {
-		.ss_sp = altstack_data,
+		/* Our sigaltstack scratch space. */
+		.ss_sp = malloc(sizeof(char) * SIGSTKSZ),
 		.ss_size = SIGSTKSZ,
 	};
 	if (sigaltstack(&stack, NULL) != 0)
@@ -872,5 +870,6 @@ int main()
 	total_nerrs += test_nonstrict_ss();
 #endif
 
+	free(stack.ss_sp);
 	return total_nerrs ? 1 : 0;
 }

tools/testing/selftests/x86/single_step_syscall.c

@@ -57,7 +57,6 @@ static void clearhandler(int sig)
 
 static volatile sig_atomic_t sig_traps, sig_eflags;
 sigjmp_buf jmpbuf;
-static unsigned char altstack_data[SIGSTKSZ];
 
 #ifdef __x86_64__
 # define REG_IP REG_RIP
@@ -210,7 +209,7 @@ int main()
 		unsigned long nr = SYS_getpid;
 		printf("[RUN]\tSet TF and check SYSENTER\n");
 		stack_t stack = {
-			.ss_sp = altstack_data,
+			.ss_sp = malloc(sizeof(char) * SIGSTKSZ),
 			.ss_size = SIGSTKSZ,
 		};
 		if (sigaltstack(&stack, NULL) != 0)
@@ -219,6 +218,7 @@ int main()
 			   SA_RESETHAND | SA_ONSTACK);
 		sethandler(SIGILL, print_and_longjmp, SA_RESETHAND);
 		set_eflags(get_eflags() | X86_EFLAGS_TF);
+		free(stack.ss_sp);
 		/* Clear EBP first to make sure we segfault cleanly. */
 		asm volatile ("xorl %%ebp, %%ebp; SYSENTER" : "+a" (nr) :: "flags", "rcx"
 #ifdef __x86_64__

tools/testing/selftests/x86/syscall_arg_fault.c

@@ -17,9 +17,6 @@
 
 #include "helpers.h"
 
-/* Our sigaltstack scratch space. */
-static unsigned char altstack_data[SIGSTKSZ];
-
 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
 		       int flags)
 {
@@ -104,7 +101,8 @@ static void sigill(int sig, siginfo_t *info, void *ctx_void)
 int main()
 {
 	stack_t stack = {
-		.ss_sp = altstack_data,
+		/* Our sigaltstack scratch space. */
+		.ss_sp = malloc(sizeof(char) * SIGSTKSZ),
 		.ss_size = SIGSTKSZ,
 	};
 	if (sigaltstack(&stack, NULL) != 0)
@@ -233,5 +231,6 @@ int main()
 	set_eflags(get_eflags() & ~X86_EFLAGS_TF);
 #endif
 
+	free(stack.ss_sp);
 	return 0;
 }