[PATCH 2/3] libdtrace: derive io provider dev_name from the device model

[Jacob Carlborg <doob@me.com>]

Without this fix, running any DTrace commands, like `dtrace -l`,
results in:

```
dtrace: invalid probe specifier :::: "/usr/lib64/dtrace/6.12.0/io.d",
line 134: failed to resolve vmlinux`major_names: Unknown symbol name
```

The issue is that on newer kernels the `major_names` symbols is not
available in `kallsyms` anymore. For example, running `grep -w
major_names /proc/kallsyms` on a newer kernel shows no results.
Running the same command on an older kernel gives the following
result:

```
$ grep -w major_names /proc/kallsyms
0000000000000000 b major_names
```

The fix is to stop depending on the `major_names` symbol and instead
derive `dev_name` from the device model (the driver name), mirroring
the `struct buffer_head` translator.

Signed-off-by: Jacob Carlborg <doob@me.com>
---
 libdtrace/io.d.in | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/libdtrace/io.d.in b/libdtrace/io.d.in
index 5ea356ee..e96a5b23 100644
--- a/libdtrace/io.d.in
+++ b/libdtrace/io.d.in
@@ -124,16 +124,27 @@ translator devinfo_t < struct buffer_head *B > {
 		       );
 };
 
+define_for_kernel([[__part0_dev]], [[(m4_kver(5,11,0), [[part0->bd_device]])]], [[part0.__dev]])
 #pragma D binding "1.6.3" translator
 translator devinfo_t < struct bio *B > {
 	dev_major = B->__disk_chk == NULL ? 0 : getmajor(B->__bio_part_dev);
 	dev_minor = B->__disk_chk == NULL ? 0 : getminor(B->__bio_part_dev);
 	dev_instance = 0;
+	/*
+	 * Derive the driver name from the device model rather than the kernel's
+	 * `major_names` array: that symbol became static and is no longer
+	 * resolvable via kallsyms on newer kernels.  This mirrors how the
+	 * struct buffer_head devinfo translator above obtains dev_name.
+	 */
 	dev_name = B->__disk_chk == NULL
 		? "nfs"
-		: stringof(((struct blk_major_name **)vmlinux`major_names)[
-				getmajor(B->__bio_part_dev) % 255
-			   ]->name);
+		: B->__disk->__part0_dev.parent
+			? B->__disk->__part0_dev.parent->driver->name
+				? stringof(B->__disk->__part0_dev.parent->driver->name)
+				: "<none>"
+			: B->__disk->__part0_dev.driver->name
+				? stringof(B->__disk->__part0_dev.driver->name)
+				: "<none>";
 	dev_statname = B->__disk_chk == NULL ? "nfs" :
 	    (B->__bio_partno) == 0 ? stringof(B->__disk->disk_name) :
 	    strjoin(stringof(B->__disk->disk_name), lltostr(B->__bio_partno));
-- 
2.50.1 (Apple Git-155)