selftests/nolibc: disable brk()/sbrk() tests on musl

On musl calls to brk() and sbrk() always fail with ENOMEM.
Detect this and skip the tests on musl.

Tested on glibc 2.39 and musl 1.2.5 in addition to nolibc.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240424-nolibc-musl-brk-v1-1-b49882dd9a93@weissschuh.net

Author: t-8ch

tools/testing/selftests/nolibc/nolibc-test.c

@@ -942,6 +942,7 @@ int run_syscall(int min, int max)
 	int ret = 0;
 	void *p1, *p2;
 	int has_gettid = 1;
+	int has_brk;
 
 	/* <proc> indicates whether or not /proc is mounted */
 	proc = stat("/proc", &stat_buf) == 0;
@@ -954,6 +955,9 @@ int run_syscall(int min, int max)
 	has_gettid = __GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 30);
 #endif
 
+	/* on musl setting brk()/sbrk() always fails */
+	has_brk = brk(0) == 0;
+
 	for (test = min; test >= 0 && test <= max; test++) {
 		int llen = 0; /* line length */
 
@@ -969,9 +973,9 @@ int run_syscall(int min, int max)
 		CASE_TEST(kill_0);            EXPECT_SYSZR(1, kill(getpid(), 0)); break;
 		CASE_TEST(kill_CONT);         EXPECT_SYSZR(1, kill(getpid(), 0)); break;
 		CASE_TEST(kill_BADPID);       EXPECT_SYSER(1, kill(INT_MAX, 0), -1, ESRCH); break;
-		CASE_TEST(sbrk_0);            EXPECT_PTRNE(1, sbrk(0), (void *)-1); break;
-		CASE_TEST(sbrk);              if ((p1 = p2 = sbrk(4096)) != (void *)-1) p2 = sbrk(-4096); EXPECT_SYSZR(1, (p2 == (void *)-1) || p2 == p1); break;
-		CASE_TEST(brk);               EXPECT_SYSZR(1, brk(sbrk(0))); break;
+		CASE_TEST(sbrk_0);            EXPECT_PTRNE(has_brk, sbrk(0), (void *)-1); break;
+		CASE_TEST(sbrk);              if ((p1 = p2 = sbrk(4096)) != (void *)-1) p2 = sbrk(-4096); EXPECT_SYSZR(has_brk, (p2 == (void *)-1) || p2 == p1); break;
+		CASE_TEST(brk);               EXPECT_SYSZR(has_brk, brk(sbrk(0))); break;
 		CASE_TEST(chdir_root);        EXPECT_SYSZR(1, chdir("/")); chdir(getenv("PWD")); break;
 		CASE_TEST(chdir_dot);         EXPECT_SYSZR(1, chdir(".")); break;
 		CASE_TEST(chdir_blah);        EXPECT_SYSER(1, chdir("/blah"), -1, ENOENT); break;