[gdb/testsuite] Add xfail in gdb.arch/i386-pkru.exp

On a x86_64-linux machine with pkru register, I run into:
...
(gdb) PASS: gdb.arch/i386-pkru.exp: set pkru value
info register pkru^M
pkru           0x12345678          305419896^M
(gdb) FAIL: gdb.arch/i386-pkru.exp: read value after setting value
...

This is a regression due to kernel commit e84ba47e313d ("x86/fpu: Hook up PKRU
onto ptrace()").  This is fixed by recent kernel commit 4a804c4f8356
("x86/fpu: Allow PKRU to be (once again) written by ptrace.").

The regression occurs for kernel versions v5.14-rc1 (the first tag containing
the regression) up to but excluding v6.2-rc1 (the first tag containing the fix).

Fix this by adding an xfail for the appropriate kernel versions.

Tested on x86_64-linux.

PR testsuite/29790
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29790

1 files changed, 47 insertions, 0 deletions

diff --git a/gdb/testsuite/lib/gdb-utils.exp b/gdb/testsuite/lib/gdb-utils.exp
index 78724f8..fb5c953 100644
--- a/gdb/testsuite/lib/gdb-utils.exp
+++ b/gdb/testsuite/lib/gdb-utils.exp
@@ -100,3 +100,50 @@ proc gdb_get_bp_addr { num } {
     }
     return ""
 }
+
+# Compare the version numbers in L1 to those in L2 using OP, and return
+# 1 if the comparison is true.
+
+proc version_compare { l1 op l2 } {
+    set len [llength $l1]
+    if { $len != [llength $l2] } {
+	error "l2 not the same length as l1"
+    }
+
+    switch -exact $op {
+	"=="    -
+	"<"     {}
+	"<="    { return [expr [version_compare $l1 < $l2] \
+			      || [version_compare $l1 == $l2]]}
+	default { error "unsupported op: $op" }
+    }
+
+    # Handle ops < and ==.
+    set idx 0
+    foreach v1 $l1 {
+	set v2 [lindex $l2 $idx]
+	incr idx
+	set last [expr $len == $idx]
+
+	set cmp [expr $v1 $op $v2]
+	if { $op == "==" } {
+	    if { $cmp } {
+		continue
+	    } else {
+		return 0
+	    }
+	} else {
+	    # $op == "<".
+	    if { $cmp } {
+		return 1
+	    } else {
+		if { !$last && $v1 == $v2 } {
+		    continue
+		}
+		return 0
+	    }
+	}
+    }
+
+    return 1
+}