kdb: Implement seq_file command

Combining the kdb seq_file infrastructure with its symbolic lookups allows
a good sub-set of files held in pseudo filesystems to be displayed by
kdb. The seq_file command does exactly this and allows a subset of pseudo
files to be safely examined whilst debugging (although in the hands of a
thoughtless development they may also hit the bigger subset that can only
be safely examined if you check the lock status first).

Good arguments to try with this command include: cpuinfo_op,
gpiolib_seq_ops and vmalloc_op.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

2 files changed, 69 insertions, 0 deletions

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index dfd3798f2b29..25c6b17c0642 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -891,6 +891,7 @@ static int __kdb_print_seq_file(struct seq_file *m, void *v)
 int kdb_print_seq_file(const struct seq_operations *ops)
 {
 	static char seq_buf[4096];
+	static char private[256] = { 0 };
 	static DEFINE_SPINLOCK(seq_buf_lock);
 	unsigned long flags;
 	struct seq_file m = {
@@ -902,6 +903,7 @@ int kdb_print_seq_file(const struct seq_operations *ops)
 		 * deadlock free with the normal interface).
 		 */
 		.op = ops,
+		.private = private,
 	};
 	loff_t pos = 0;
 	void *v;
diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 0852a537dad4..4edbe81f879a 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -1775,6 +1775,67 @@ static int kdb_mm(int argc, const char **argv)
 	return 0;
 }
 
+static int kdb_validate_function_pointer(unsigned long addr)
+{
+	kdb_symtab_t sym;
+	int found;
+
+	found = kdbnearsym(addr, &sym);
+	if (!found || sym.sym_start != addr) {
+		kdb_printf("Bad function pointer 0x%lx\n", addr);
+		return KDB_BADADDR;
+	}
+
+	return 0;
+}
+
+/*
+ * kdb_seq_file - This function implements the 'seq_file' command.
+ *	seq_file address-expression
+ * Remarks:
+ *	We cannot fully validate the user supplied pointer (in particular we
+ *	cannot check that the locking is safe). However we will do our best
+ *	to check it really is an operations table since, if it isn't, we'll
+ *	probably panic the kernel when we follow those function pointers.
+ */
+static int kdb_seq_file(int argc, const char **argv)
+{
+	int diag;
+	unsigned long addr;
+	int nextarg;
+	long offset;
+	char *name;
+	const struct seq_operations *ops;
+	struct seq_operations ops_copy;
+
+	nextarg = 1;
+	diag = kdbgetaddrarg(argc, argv, &nextarg, &addr, &offset, &name);
+	if (diag)
+		return diag;
+
+	if (nextarg != argc+1)
+		return KDB_ARGCOUNT;
+
+	ops = (const struct seq_operations *) (addr + offset);
+
+	/* Check we can read the pointer address OK */
+	if (kdb_getarea(ops_copy, (unsigned long) ops)) {
+		kdb_printf("Cannot access sequence ops at 0x%px\n", ops);
+		return KDB_BADADDR;
+	}
+
+	kdb_printf("Using sequence_ops at 0x%px (%s)\n", ops, name);
+
+	/* Validate the function pointers we expect to find at this address */
+	if (kdb_validate_function_pointer((unsigned long) ops->start) ||
+	    kdb_validate_function_pointer((unsigned long) ops->next) ||
+	    kdb_validate_function_pointer((unsigned long) ops->stop) ||
+	    kdb_validate_function_pointer((unsigned long) ops->show))
+		return KDB_BADADDR;
+
+	return kdb_print_seq_file(ops);
+}
+
 /*
  * kdb_go - This function implements the 'go' command.
  *	go [address-expression]
@@ -2875,6 +2936,12 @@ static kdbtab_t maintab[] = {
 		.help = "Display help on | grep",
 		.flags = KDB_ENABLE_ALWAYS_SAFE,
 	},
+	{	.name = "seq_file",
+		.func = kdb_seq_file,
+		.usage = "<seq_ops>",
+		.help = "Show a seq_file using struct seq_operations",
+		.flags = KDB_ENABLE_REG_WRITE,
+	},
 };
 
 static kdbtab_t nmicmd = {