linux-kernel - Re: [PATCH] ntb: idt: fix clang -Wformat warnings

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Date:   Tue, 12 Jul 2022 14:40:15 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Jon Mason <jdmason@...zu.us>, Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, ntb@...ts.linux.dev,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] ntb: idt: fix clang -Wformat warnings

Hi Justin

On Mon, Jul 11, 2022 at 04:01:48PM -0700, Justin Stitt wrote:
> When building with Clang we encounter these warnings:
> | drivers/ntb/hw/idt/ntb_hw_idt.c:2409:28: error: format specifies type
> | 'unsigned char' but the argument has type 'int' [-Werror,-Wformat]
> | "\t%hhu-%hhu.\t", idx + cnt - 1);
> -
> | drivers/ntb/hw/idt/ntb_hw_idt.c:2438:29: error: format specifies type
> | 'unsigned char' but the argument has type 'int' [-Werror,-Wformat]
> | "\t%hhu-%hhu.\t", idx + cnt - 1);
> -
> | drivers/ntb/hw/idt/ntb_hw_idt.c:2484:15: error: format specifies type
> | 'unsigned char' but the argument has type 'int' [-Werror,-Wformat], src);
> 
> For the first two warnings the format specifier used is `%hhu` which
> describes a u8. Both `idx` and `cnt` are u8 as well. However, the
> expression as a whole is promoted to an int as you cannot get
> smaller-than-int from addition. Therefore, to fix the warning, use the
> promoted-to-type's format specifier -- in this case `%d`.
> 
> example:
> ``
> uint8_t a = 4, b = 7;
> int size = sizeof(a + b - 1);
> printf("%d\n", size);
> // output: 4
> ```
> 
> For the last warning, src is of type `int` while the format specifier
> describes a u8. The fix here is just to use the proper specifier `%d`.
> 
> See more:
> (https://wiki.sei.cmu.edu/confluence/display/c/INT02-C.+Understand+integer+conversion+rules)
> "Integer types smaller than int are promoted when an operation is
> performed on them. If all values of the original type can be represented
> as an int, the value of the smaller type is converted to an int;
> otherwise, it is converted to an unsigned int."
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/378
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Thanks for fixing this. Definitely
Acked-by: Serge Semin <fancer.lancer@...il.com>

-Sergey

> ---
> Note: This patch silences the -Wformat warnings for this file (which is
> the goal) but in reality all instances of `%hh[dux]` should be converted
> to `%[dux]` for this file and probably every file. That's a bit larger
> scope than the goal of enabling -Wformat for Clang builds, though.
> 
>  drivers/ntb/hw/idt/ntb_hw_idt.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/ntb/hw/idt/ntb_hw_idt.c b/drivers/ntb/hw/idt/ntb_hw_idt.c
> index 733557231ed0..0ed6f809ff2e 100644
> --- a/drivers/ntb/hw/idt/ntb_hw_idt.c
> +++ b/drivers/ntb/hw/idt/ntb_hw_idt.c
> @@ -2406,7 +2406,7 @@ static ssize_t idt_dbgfs_info_read(struct file *filp, char __user *ubuf,
>  				"\t%hhu.\t", idx);
>  		else
>  			off += scnprintf(strbuf + off, size - off,
> -				"\t%hhu-%hhu.\t", idx, idx + cnt - 1);
> +				"\t%hhu-%d.\t", idx, idx + cnt - 1);
>  
>  		off += scnprintf(strbuf + off, size - off, "%s BAR%hhu, ",
>  			idt_get_mw_name(data), ndev->mws[idx].bar);
> @@ -2435,7 +2435,7 @@ static ssize_t idt_dbgfs_info_read(struct file *filp, char __user *ubuf,
>  					"\t%hhu.\t", idx);
>  			else
>  				off += scnprintf(strbuf + off, size - off,
> -					"\t%hhu-%hhu.\t", idx, idx + cnt - 1);
> +					"\t%hhu-%d.\t", idx, idx + cnt - 1);
>  
>  			off += scnprintf(strbuf + off, size - off,
>  				"%s BAR%hhu, ", idt_get_mw_name(data),
> @@ -2480,7 +2480,7 @@ static ssize_t idt_dbgfs_info_read(struct file *filp, char __user *ubuf,
>  		int src;
>  		data = idt_ntb_msg_read(&ndev->ntb, &src, idx);
>  		off += scnprintf(strbuf + off, size - off,
> -			"\t%hhu. 0x%08x from peer %hhu (Port %hhu)\n",
> +			"\t%hhu. 0x%08x from peer %d (Port %hhu)\n",
>  			idx, data, src, ndev->peers[src].port);
>  	}
>  	off += scnprintf(strbuf + off, size - off, "\n");
> -- 
> 2.37.0.144.g8ac04bfd2-goog
>