selftests/seccomp: Don't call read() on TTY from background pgrp

Since commit 92d25637 ("kselftest: signal all child processes"), tests
are executed in background process groups. This means that trying to read
from stdin now throws SIGTTIN when stdin is a TTY, which breaks some
seccomp selftests that try to use read(0, NULL, 0) as a dummy syscall.

The simplest way to fix that is probably to just use -1 instead of 0 as
the dummy read()'s FD.

Fixes: 92d25637 ("kselftest: signal all child processes")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220319010011.1374622-1-jannh@google.com

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -955,7 +955,7 @@ TEST(ERRNO_valid)
 	ASSERT_EQ(0, ret);
 
 	EXPECT_EQ(parent, syscall(__NR_getppid));
-	EXPECT_EQ(-1, read(0, NULL, 0));
+	EXPECT_EQ(-1, read(-1, NULL, 0));
 	EXPECT_EQ(E2BIG, errno);
 }
 
@@ -974,7 +974,7 @@ TEST(ERRNO_zero)
 
 	EXPECT_EQ(parent, syscall(__NR_getppid));
 	/* "errno" of 0 is ok. */
-	EXPECT_EQ(0, read(0, NULL, 0));
+	EXPECT_EQ(0, read(-1, NULL, 0));
 }
 
 /*
@@ -995,7 +995,7 @@ TEST(ERRNO_capped)
 	ASSERT_EQ(0, ret);
 
 	EXPECT_EQ(parent, syscall(__NR_getppid));
-	EXPECT_EQ(-1, read(0, NULL, 0));
+	EXPECT_EQ(-1, read(-1, NULL, 0));
 	EXPECT_EQ(4095, errno);
 }
 
@@ -1026,7 +1026,7 @@ TEST(ERRNO_order)
 	ASSERT_EQ(0, ret);
 
 	EXPECT_EQ(parent, syscall(__NR_getppid));
-	EXPECT_EQ(-1, read(0, NULL, 0));
+	EXPECT_EQ(-1, read(-1, NULL, 0));
 	EXPECT_EQ(12, errno);
 }
 
@@ -2623,7 +2623,7 @@ void *tsync_sibling(void *data)
 	ret = prctl(PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0);
 	if (!ret)
 		return (void *)SIBLING_EXIT_NEWPRIVS;
-	read(0, NULL, 0);
+	read(-1, NULL, 0);
 	return (void *)SIBLING_EXIT_UNKILLED;
 }