linux-kernel - [PATCH 5.15 065/189] gpiolib: acpi: use correct format characters

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Date:   Mon, 18 Apr 2022 14:11:25 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 065/189] gpiolib: acpi: use correct format characters

From: Linus Torvalds <torvalds@...ux-foundation.org>

[ Upstream commit 213d266ebfb1621aab79cfe63388facc520a1381 ]

When compiling with -Wformat, clang emits the following warning:

  gpiolib-acpi.c:393:4: warning: format specifies type 'unsigned char' but the argument has type 'int' [-Wformat]
                        pin);
                        ^~~

So warning that '%hhX' is paired with an 'int' is all just completely
mindless and wrong. Sadly, I can see a different bogus warning reason
why people would want to use '%02hhX'.

Again, the *sane* thing from a human perspective is to use '%02X. But
if the compiler doesn't do any range analysis at all, it could decide
that "Oh, that print format could need up to 8 bytes of space in the
result". Using '%02hhX' would cut that down to two.

And since we use

        char ev_name[5];

and currently use "_%c%02hhX" as the format string, even a compiler
that doesn't notice that "pin <= 255" test that guards this all will
go "OK, that's at most 4 bytes and the final NUL termination, so it's
fine".

While a compiler - like gcc - that only sees that the original source
of the 'pin' value is a 'unsigned short' array, and then doesn't take
the "pin <= 255" into account, will warn like this:

  gpiolib-acpi.c: In function 'acpi_gpiochip_request_interrupt':
  gpiolib-acpi.c:206:24: warning: '%02X' directive writing between 2 and 4 bytes into a region of size 3 [-Wformat-overflow=]
       sprintf(ev_name, "_%c%02X",
                            ^~~~
  gpiolib-acpi.c:206:20: note: directive argument in the range [0, 65535]

because gcc isn't being very good at that argument range analysis either.

In other words, the original use of 'hhx' was bogus to begin with, and
due to *another* compiler warning being bad, and we had that bad code
being written back in 2016 to work around _that_ compiler warning
(commit e40a3ae1f794: "gpio: acpi: work around false-positive
-Wstring-overflow warning").

Sadly, two different bad compiler warnings together does not make for
one good one.

It just makes for even more pain.

End result: I think the simplest and cleanest option is simply the
proposed change which undoes that '%hhX' change for gcc, and replaces
it with just using a slightly bigger stack allocation. It's not like
a 5-byte allocation is in any way likely to have saved any actual stack,
since all the other variables in that function are 'int' or bigger.

False-positive compiler warnings really do make people write worse
code, and that's a problem. But on a scale of bad code, I feel that
extending the buffer trivially is better than adding a pointless cast
that literally makes no sense.

At least in this case the end result isn't unreadable or buggy. We've
had several cases of bad compiler warnings that caused changes that
were actually horrendously wrong.

Fixes: e40a3ae1f794 ("gpio: acpi: work around false-positive -Wstring-overflow warning")
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/gpio/gpiolib-acpi.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpio/gpiolib-acpi.c b/drivers/gpio/gpiolib-acpi.c
index 4c2e32c38acc..53be0bdf2bc3 100644
--- a/drivers/gpio/gpiolib-acpi.c
+++ b/drivers/gpio/gpiolib-acpi.c
@@ -392,8 +392,8 @@ static acpi_status acpi_gpiochip_alloc_event(struct acpi_resource *ares,
 	pin = agpio->pin_table[0];

 	if (pin <= 255) {
-		char ev_name[5];
-		sprintf(ev_name, "_%c%02hhX",
+		char ev_name[8];
+		sprintf(ev_name, "_%c%02X",
 			agpio->triggering == ACPI_EDGE_SENSITIVE ? 'E' : 'L',
 			pin);
 		if (ACPI_SUCCESS(acpi_get_handle(handle, ev_name, &evt_handle)))
-- 
2.35.1