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

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Message-ID: <YOIpM3iFT5roz69i@casper.infradead.org>
Date:   Sun, 4 Jul 2021 22:33:39 +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 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.


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");