string_helpers: Escape double quotes in escape_special

From an abstract point of view, escape_special's counterpart,
unescape_special, already handles the unescaping of blackslashed double
quote sequences.

As a more practical example, printk indexing is an example case where
this is already practically useful. Compare an example with
`ESCAPE_SPECIAL | ESCAPE_SPACE`, with quotes not escaped:

    [root@ktst ~]# grep drivers/pci/pci-stub.c:69 /sys/kernel/debug/printk/index/vmlinux
    <4> drivers/pci/pci-stub.c:69 pci_stub_init "pci-stub: invalid ID string "%s"\n"

...and the same after this patch:

    [root@ktst ~]# grep drivers/pci/pci-stub.c:69 /sys/kernel/debug/printk/index/vmlinux
    <4> drivers/pci/pci-stub.c:69 pci_stub_init "pci-stub: invalid ID string \"%s\"\n"

One can of course, alternatively, use ESCAPE_APPEND with a quote in
@only, but without this patch quotes are coerced into hex or octal which
can hurt readability quite significantly.

I've checked uses of ESCAPE_SPECIAL and %pE across the codebase, and I'm
pretty confident that this shouldn't affect any stable interfaces.

Signed-off-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/af144c5b75e41ce417386253ba2694456bc04118.1623775748.git.chris@chrisdown.name

lib/string_helpers.c

@@ -361,6 +361,9 @@ static bool escape_special(unsigned char c, char **dst, char *end)
 	case '\e':
 		to = 'e';
 		break;
+	case '"':
+		to = '"';
+		break;
 	default:
 		return false;
 	}
@@ -474,6 +477,7 @@ static bool escape_hex(unsigned char c, char **dst, char *end)
  *		'\t' - horizontal tab
  *		'\v' - vertical tab
  *	%ESCAPE_SPECIAL:
+ *		'\"' - double quote
  *		'\\' - backslash
  *		'\a' - alert (BEL)
  *		'\e' - escape

lib/test-string_helpers.c

@@ -140,13 +140,13 @@ static const struct test_string_2 escape0[] __initconst = {{
 },{
 	.in = "\\h\\\"\a\e\\",
 	.s1 = {{
-		.out = "\\\\h\\\\\"\\a\\e\\\\",
+		.out = "\\\\h\\\\\\\"\\a\\e\\\\",
 		.flags = ESCAPE_SPECIAL,
 	},{
-		.out = "\\\\\\150\\\\\\042\\a\\e\\\\",
+		.out = "\\\\\\150\\\\\\\"\\a\\e\\\\",
 		.flags = ESCAPE_SPECIAL | ESCAPE_OCTAL,
 	},{
-		.out = "\\\\\\x68\\\\\\x22\\a\\e\\\\",
+		.out = "\\\\\\x68\\\\\\\"\\a\\e\\\\",
 		.flags = ESCAPE_SPECIAL | ESCAPE_HEX,
 	},{
 		/* terminator */
@@ -157,10 +157,10 @@ static const struct test_string_2 escape0[] __initconst = {{
 		.out = "\eb \\C\007\"\x90\\r]",
 		.flags = ESCAPE_SPACE,
 	},{
-		.out = "\\eb \\\\C\\a\"\x90\r]",
+		.out = "\\eb \\\\C\\a\\\"\x90\r]",
 		.flags = ESCAPE_SPECIAL,
 	},{
-		.out = "\\eb \\\\C\\a\"\x90\\r]",
+		.out = "\\eb \\\\C\\a\\\"\x90\\r]",
 		.flags = ESCAPE_SPACE | ESCAPE_SPECIAL,
 	},{
 		.out = "\\033\\142\\040\\134\\103\\007\\042\\220\\015\\135",
@@ -169,10 +169,10 @@ static const struct test_string_2 escape0[] __initconst = {{
 		.out = "\\033\\142\\040\\134\\103\\007\\042\\220\\r\\135",
 		.flags = ESCAPE_SPACE | ESCAPE_OCTAL,
 	},{
-		.out = "\\e\\142\\040\\\\\\103\\a\\042\\220\\015\\135",
+		.out = "\\e\\142\\040\\\\\\103\\a\\\"\\220\\015\\135",
 		.flags = ESCAPE_SPECIAL | ESCAPE_OCTAL,
 	},{
-		.out = "\\e\\142\\040\\\\\\103\\a\\042\\220\\r\\135",
+		.out = "\\e\\142\\040\\\\\\103\\a\\\"\\220\\r\\135",
 		.flags = ESCAPE_SPACE | ESCAPE_SPECIAL | ESCAPE_OCTAL,
 	},{
 		.out = "\eb \\C\007\"\x90\r]",