linux-kernel - Re: [PATCH] eeprom: idt_89hpesx: fix clang -Wformat warnings

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Date:   Thu, 7 Jul 2022 11:03:49 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] eeprom: idt_89hpesx: fix clang -Wformat warnings

On Fri, Jul 01, 2022 at 04:20:31PM -0700, Justin Stitt wrote:
> see warnings:
> | drivers/misc/eeprom/idt_89hpesx.c:570:5: error: format specifies type
> | 'unsigned char' but the argument has type 'u16' (aka 'unsigned short')
> | [-Werror,-Wformat] memaddr);
> -
> | drivers/misc/eeprom/idt_89hpesx.c:579:5: error: format specifies type
> | 'unsigned char' but the argument has type 'u16' (aka 'unsigned short')
> | [-Werror,-Wformat] memaddr);
> -
> | drivers/misc/eeprom/idt_89hpesx.c:814:4: error: format specifies type
> | 'unsigned short' but the argument has type 'unsigned int'
> | [-Werror,-Wformat] CSR_REAL_ADDR(csraddr));
> 
> There's an ongoing movement to eventually enable the -Wformat flag for
> clang. See: https://github.com/ClangBuiltLinux/linux/issues/378
> 
> The format specifier for idt_89hpesx.c:570 and 579 was `0x%02hhx`. The
> part we care about `%hhx` describes a single byte format, wherein the
> leftmost byte of our u16 type (of which memaddr is) is truncated.
> 
> example:
> ```
> uint16_t x = 0xbabe;
> printf("%hhx\n", x);
> // output is: be
> // we lost 'ba'
> ```
> 
> There exists a similar issue at idt_89hpesx.c:814 which involves the
> CSR_REAL_ADDR macro. This macro returns a u16 but due to default
> argument promotion for variadic functions (printf-like) actually
> provides an int to the dev_err method.
> 
> My proposed solution is to expand the width of the format specifier to
> fully encompass the provided argument (which is promoted to an int, see
> below). I opted for '%x' as this specifies an unsigned hexadecimal
> integer which, with a guarantee, can represent all the values of a u16.

Additionally, this solution is in line with printk-formats.rst after
commit cbacb5ab0aa0 ("docs: printk-formats: Stop encouraging use of
unnecessary %h[xudi] and %hh[xudi]").

> As per C11 6.3.1.1:
> (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf)
> `If an int can represent all values of the original type ..., the
> value is converted to an int; otherwise, it is converted to an
> unsigned int. These are called the integer promotions.`
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/378
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  drivers/misc/eeprom/idt_89hpesx.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/misc/eeprom/idt_89hpesx.c b/drivers/misc/eeprom/idt_89hpesx.c
> index b0cff4b152da..42eac114edd7 100644
> --- a/drivers/misc/eeprom/idt_89hpesx.c
> +++ b/drivers/misc/eeprom/idt_89hpesx.c
> @@ -566,7 +566,7 @@ static int idt_eeprom_read_byte(struct idt_89hpesx_dev *pdev, u16 memaddr,
>  		eeseq.memaddr = cpu_to_le16(memaddr);
>  		ret = pdev->smb_write(pdev, &smbseq);
>  		if (ret != 0) {
> -			dev_err(dev, "Failed to init eeprom addr 0x%02hhx",
> +			dev_err(dev, "Failed to init eeprom addr 0x%02x",
>  				memaddr);
>  			break;
>  		}
> @@ -575,7 +575,7 @@ static int idt_eeprom_read_byte(struct idt_89hpesx_dev *pdev, u16 memaddr,
>  		smbseq.bytecnt = EEPROM_RD_CNT;
>  		ret = pdev->smb_read(pdev, &smbseq);
>  		if (ret != 0) {
> -			dev_err(dev, "Failed to read eeprom data 0x%02hhx",
> +			dev_err(dev, "Failed to read eeprom data 0x%02x",
>  				memaddr);
>  			break;
>  		}
> @@ -810,7 +810,7 @@ static int idt_csr_read(struct idt_89hpesx_dev *pdev, u16 csraddr, u32 *data)
>  	smbseq.bytecnt = CSR_RD_CNT;
>  	ret = pdev->smb_read(pdev, &smbseq);
>  	if (ret != 0) {
> -		dev_err(dev, "Failed to read csr 0x%04hx",
> +		dev_err(dev, "Failed to read csr 0x%04x",
>  			CSR_REAL_ADDR(csraddr));
>  		goto err_mutex_unlock;
>  	}
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
>