[LTP] [PATCH v2] ldt.h: Add workaround for x86_64

[Ricardo B. Marlière]

From: Ricardo B. Marlière <rbm@suse.com>

The commit be0aaca2f742 ("syscalls/modify_ldt: Add lapi/ldt.h") left behind
an important factor of modify_ldt(): the kernel intentionally casts the
return value to unsigned int. This was handled in
testcases/cve/cve-2015-3290.c but was removed. Add it back to the relevant
file.

Reported-by: Martin Doucha <mdoucha@suse.cz>
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
---
Changes in v2:
- Added TBROK for any ret != 0 in modify_ldt call in cve-2015-3290.c
- Link to v1: https://lore.kernel.org/r/20250507-fixes-modify_ldt-v1-1-70a2694cfddc@suse.com
---
 include/lapi/ldt.h            | 22 +++++++++++++++++++++-
 testcases/cve/cve-2015-3290.c |  8 +++++++-
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/include/lapi/ldt.h b/include/lapi/ldt.h
index 6b5a2d59cb41bfc24eb5ac26c3d47d49fb8ff78f..173321dd9ac34ba87eff0eee960635f30d878991 100644
--- a/include/lapi/ldt.h
+++ b/include/lapi/ldt.h
@@ -31,7 +31,27 @@ struct user_desc {
 static inline int modify_ldt(int func, const struct user_desc *ptr,
 			     unsigned long bytecount)
 {
-	return tst_syscall(__NR_modify_ldt, func, ptr, bytecount);
+	long rval;
+
+	errno = 0;
+	rval = tst_syscall(__NR_modify_ldt, func, ptr, bytecount);
+
+#ifdef __x86_64__
+	/*
+	 * The kernel intentionally casts modify_ldt() return value
+	 * to unsigned int to prevent sign extension to 64 bits. This may
+	 * result in syscall() returning the value as is instead of setting
+	 * errno and returning -1.
+	 */
+	if (rval > 0 && (int)rval < 0) {
+		tst_res(TINFO,
+			"WARNING: Libc mishandled modify_ldt() return value");
+		errno = -(int)errno;
+		rval = -1;
+	}
+#endif /* __x86_64__ */
+
+	return rval;
 }
 
 static inline int safe_modify_ldt(const char *file, const int lineno, int func,
diff --git a/testcases/cve/cve-2015-3290.c b/testcases/cve/cve-2015-3290.c
index 8ec1d53bbb5a9f3e7761d39855d34f593e118a28..e70742acc87c39088953e02f16146b7b58a75fd1 100644
--- a/testcases/cve/cve-2015-3290.c
+++ b/testcases/cve/cve-2015-3290.c
@@ -197,7 +197,13 @@ static void set_ldt(void)
 		.useable	 = 0
 	};
 
-	SAFE_MODIFY_LDT(1, &data_desc, sizeof(data_desc));
+	TEST(modify_ldt(1, &data_desc, sizeof(data_desc)));
+	if (TST_RET == -1 && TST_ERR == EINVAL) {
+		tst_brk(TCONF | TTERRNO,
+			"modify_ldt: 16-bit data segments are probably disabled");
+	} else if (TST_RET != 0) {
+		tst_brk(TBROK | TTERRNO, "modify_ldt");
+	}
 }
 
 static void try_corrupt_stack(unsigned short *orig_ss)

---
base-commit: b070a5692e035ec12c3d3c7a7e9e97c270fd4d7d
change-id: 20250507-fixes-modify_ldt-ebcfdd2a7d30

Best regards,
-- 
Ricardo B. Marlière <rbm@suse.com>