linux-kernel - Re: [PATCH 01/17] kallsyms: support big kernel symbols (2-byte lengths)

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Date:   Sun, 4 Jul 2021 22:49:09 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux <rust-for-linux@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Geoffrey Thomas <geofft@...reload.com>,
        Finn Behrens <me@...enk.de>,
        Adam Bratschi-Kaye <ark.email@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>
Subject: Re: [PATCH 01/17] kallsyms: support big kernel symbols (2-byte
 lengths)

On Sun, Jul 04, 2021 at 10:33:39PM +0100, Matthew Wilcox wrote:
> On Sun, Jul 04, 2021 at 11:17:50PM +0200, Miguel Ojeda wrote:
> > On Sun, Jul 4, 2021 at 11:05 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > What happened to my suggestion from last time of encoding symbols < 128
> > > as 0-127 and symbols larger than that as (data[0] - 128) * 256 +
> > > data[1]) ?
> > 
> > Nothing, sorry, we focused on other parts (e.g. the allocation panics)
> > during this iteration. I can take a look for v2.
> 
> Here's what I have.  Build testing now.

It seems to work.  Sample output from kallsyms with the test forced to
'true':

        .byte 0x80
        .byte 0x0a, 0x41, 0xff, 0x70, 0xf3, 0xd0, 0xb0, 0xf2, 0xfc, 0x72, 0x74

under normal circumstances, it omits that first line.

I'm unfortunately in the middle of ripping apart and fixing up my test
infrastructure, so I can't claim to have really tested it, and I don't
have a module with a really large symbol anyway.  But I'll submit this
to you as a proper patch with changelog if that's enough testing for you.

(now that i look at kallsyms, why on earth is it ripping the string
apart and turning it into .byte instead of just using .ascii?)

> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index c851ca0ed357..0d45a6e5fdc3 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -67,6 +67,14 @@ static unsigned int kallsyms_expand_symbol(unsigned int off,
>  	len = *data;
>  	data++;
>  
> +	/* lengths larger than 128 are encoded as two bytes */
> +	if (len >= 128) {
> +		len -= 128;
> +		len *= 256;
> +		len += *data;
> +		data++;
> +	}
> +
>  	/*
>  	 * Update the offset to return the offset for the next symbol on
>  	 * the compressed stream.
> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> index 54ad86d13784..701566e01a1d 100644
> --- a/scripts/kallsyms.c
> +++ b/scripts/kallsyms.c
> @@ -467,10 +467,16 @@ static void write_src(void)
>  	output_label("kallsyms_names");
>  	off = 0;
>  	for (i = 0; i < table_cnt; i++) {
> +		int len = table[i]->len;
>  		if ((i & 0xFF) == 0)
>  			markers[i >> 8] = off;
>  
> -		printf("\t.byte 0x%02x", table[i]->len);
> +		if (len >= 128) {
> +			printf("\t.byte 0x%02x\n", len / 256 + 128);
> +			len %= 256;
> +			off++;
> +		}
> +		printf("\t.byte 0x%02x", len);
>  		for (k = 0; k < table[i]->len; k++)
>  			printf(", 0x%02x", table[i]->sym[k]);
>  		printf("\n");