linux-kernel - [BUG] Argument-alignment build error with clang

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Message-ID: <38508cf1-7d44-4656-8060-973e820b2957@paulmck-laptop>
Date: Wed, 23 Oct 2024 15:26:57 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: kobak@...dia.com
Cc: mochs@...dia.com, rui.zhang@...el.com, ardb@...nel.org,
	rafael.j.wysocki@...el.com, sfr@...b.auug.org.au,
	linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
	linux-toolchains@...r.kernel.org
Subject: [BUG] Argument-alignment build error with clang

Hello!

Running rcutorture on next-20241023 got me lots of these:

drivers/acpi/prmt.c:156:29: error: passing 1-byte aligned argument to 4-byte aligned parameter 1 of 'efi_pa_va_lookup' may result in an unaligned pointer access [-Werror,-Walign-mismatch]
          156 |                         (void *)efi_pa_va_lookup(&th->guid, handler_info->handler_address);

This is built with CC=clang.  I don't see this diagnostic with GCC.
But we are supposed to be able to build with clang, so...

The first argument is the address of one of these:

typedef struct {
	__u8 b[UUID_SIZE];
} guid_t;

Where UUID_SIZE is as follows:

#define UUID_SIZE 16

But this guid_t is a member of one of these:

struct prm_handler_info {
	guid_t guid;
	efi_status_t (__efiapi *handler_addr)(u64, void *);
	u64 static_data_buffer_addr;
	u64 acpi_param_buffer_addr;

	struct list_head handler_list;
};

One can argue that this structure must be 16-bit aligned on a
64-bit build.  So maybe this is a bug in clang's diagnostics, hence
linux-toolchains on CC.

Thoughts?

						Thanx, Paul