[igt-dev] [PATCH i-g-t] lib/kmod: reimplement kmsg_dump()

[Lucas De Marchi]

/dev/kmsg is record-oriented rather than line-oriented. A short read in
which the record doesn't fit will return -EINVAL and getline() will stop
working. This is in general not a problem since the (default) stdio
buffer is bigger than the maximum record size (4096). However let's
abide by the kernel ABI rather than relying on the libc buffer size.

While reimplementing, fix the following issues that made me look to the
implementation in the first place:

1) we were skipping 1 char in the message, producing messages like the
   one below (see the missing i in intel_pch_type):
	(i915_selftest:3861) igt_kmod-WARNING: ntel_pch_type [i915]] Found Tiger Lake LP PCH

2) we were printing the key=val pair of kmsg as if it was part of the
messsages, and even mangling the result looking for a ':':
	(i915_selftest:3861) igt_kmod-WARNING: 000:00:02.0

The ABI for /dev/kmsg stands:
	Accessing the buffer:
		Every read() from the opened device node receives one record
		of the kernel's printk buffer.

		[ ... ]

		Future extensions might add more comma separated values before
		the terminating ';'. Unknown fields and values should be
		gracefully ignored.

		The human readable text string starts directly after the ';'
		and is terminated by a '\n'. Untrusted values derived from
		hardware or other facilities are printed, therefore
		all non-printable characters and '\' itself in the log message
		are escaped by "\x00" C-style hex encoding.

		A line starting with ' ', is a continuation line, adding
		key/value pairs to the log message, which provide the machine
		readable context of the message, for reliable processing in
		userspace.

Now the line in (1) print as:
	(i915_selftest:5070) igt_kmod-WARNING: [drm:intel_pch_type [i915]] Found Tiger Lake LP PCH

This also fixes a double close on fclose() already closes the file
descriptor.

Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
---
 lib/igt_kmod.c | 43 ++++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/lib/igt_kmod.c b/lib/igt_kmod.c
index 91662eb3..01faca39 100644
--- a/lib/igt_kmod.c
+++ b/lib/igt_kmod.c
@@ -374,25 +374,34 @@ igt_i915_driver_unload(void)
 
 static void kmsg_dump(int fd)
 {
-	FILE *file;
-
-	file = NULL;
-	if (fd != -1)
-		file = fdopen(fd, "r");
-	if (file) {
-		size_t len = 0;
-		char *line = NULL;
-
-		while (getline(&line, &len, file) != -1) {
-			char *start = strchr(line, ':');
-			if (start)
-				igt_warn("%s", start + 2);
-		}
+	char record[4096 + 1];
 
-		free(line);
-		fclose(file);
-	} else {
+	if (fd == -1) {
 		igt_warn("Unable to retrieve kernel log (from /dev/kmsg)\n");
+		return;
+	}
+
+	record[sizeof(record) - 1] = '\0';
+
+	for (;;) {
+		const char *start, *end;
+		ssize_t r;
+
+		r = read(fd, record, sizeof(record) - 1);
+		if (r < 0) {
+			if (errno == EINTR)
+				continue;
+			if (errno != EAGAIN)
+				igt_warn("kmsg truncated due to unknown error: %m\n");
+			break;
+		}
+
+		start = strchr(record, ';');
+		if (start) {
+			start++;
+			end = strchrnul(start, '\n');
+			igt_warn("%.*s\n", (int)(end - start), start);
+		}
 	}
 }
 
-- 
2.23.0