Re: [PATCH] modpost: support arbitrary symbol length in modversion

[Lucas De Marchi <lucas.demarchi@intel.com>]

On Wed, Jan 11, 2023 at 04:11:51PM +0000, Gary Guo wrote:
>Currently modversion uses a fixed size array of size (64 - sizeof(long))
>to store symbol names, thus placing a hard limit on length of symbols.
>Rust symbols (which encodes crate and module names) can be quite a bit
>longer. The length limit in kallsyms is increased to 512 for this reason.
>
>It's a waste of space to simply expand the fixed array size to 512 in
>modversion info entries. I therefore make it variably sized, with offset
>to the next entry indicated by the initial "next" field.
>
>In addition to supporting longer-than-56/60 byte symbols, this patch also
>reduce the size for short symbols by getting rid of excessive 0 paddings.
>There are still some zero paddings to ensure "next" and "crc" fields are
>properly aligned.
>
>This patch does have a tiny drawback that it makes ".mod.c" files generated
>a bit less easy to read, as code like
>
>	"\x08\x00\x00\x00\x78\x56\x34\x12"
>	"symbol\0\0"
>
>is generated as opposed to
>
>	{ 0x12345678, "symbol" },
>
>because the structure is now variable-length. But hopefully nobody reads
>the generated file :)
>
>Link: b8a94bfb3395 ("kallsyms: increase maximum kernel symbol length to 512")
>Link: https://github.com/Rust-for-Linux/linux/pull/379
>
>Signed-off-by: Gary Guo <gary@garyguo.net>
>---
> arch/powerpc/kernel/module_64.c |  3 ++-
> include/linux/module.h          |  6 ++++--
> kernel/module/version.c         | 21 +++++++++------------
> scripts/export_report.pl        |  9 +++++----
> scripts/mod/modpost.c           | 33 +++++++++++++++++++++++----------
> 5 files changed, 43 insertions(+), 29 deletions(-)
>
>diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
>index ff045644f13f..eac23c11d579 100644
>--- a/arch/powerpc/kernel/module_64.c
>+++ b/arch/powerpc/kernel/module_64.c
>@@ -236,10 +236,11 @@ static void dedotify_versions(struct modversion_info *vers,
> {
> 	struct modversion_info *end;
>
>-	for (end = (void *)vers + size; vers < end; vers++)
>+	for (end = (void *)vers + size; vers < end; vers = (void *)vers + vers->next) {
> 		if (vers->name[0] == '.') {
> 			memmove(vers->name, vers->name+1, strlen(vers->name));
> 		}
>+	}
> }
>
> /*
>diff --git a/include/linux/module.h b/include/linux/module.h
>index 8c5909c0076c..37cb25af9099 100644
>--- a/include/linux/module.h
>+++ b/include/linux/module.h
>@@ -34,8 +34,10 @@
> #define MODULE_NAME_LEN MAX_PARAM_PREFIX_LEN
>
> struct modversion_info {
>-	unsigned long crc;
>-	char name[MODULE_NAME_LEN];
>+	/* Offset of the next modversion entry in relation to this one. */
>+	u32 next;
>+	u32 crc;
>+	char name[0];

although not really exported as uapi, this will break userspace as this is
used in the  elf file generated for the modules. I think
this change must be made in a backward compatible way and kmod updated
to deal with the variable name length:

kmod $ git grep "\[64"
libkmod/libkmod-elf.c:  char name[64 - sizeof(uint32_t)];
libkmod/libkmod-elf.c:  char name[64 - sizeof(uint64_t)];

in kmod we have both 32 and 64 because a 64-bit kmod can read both 32
and 64 bit module, and vice versa.

Lucas De Marchi

> };
>
> struct module;
>diff --git a/kernel/module/version.c b/kernel/module/version.c
>index 53f43ac5a73e..af7478dcc158 100644
>--- a/kernel/module/version.c
>+++ b/kernel/module/version.c
>@@ -17,32 +17,29 @@ int check_version(const struct load_info *info,
> {
> 	Elf_Shdr *sechdrs = info->sechdrs;
> 	unsigned int versindex = info->index.vers;
>-	unsigned int i, num_versions;
>-	struct modversion_info *versions;
>+	struct modversion_info *versions, *end;
>+	u32 crcval;
>
> 	/* Exporting module didn't supply crcs?  OK, we're already tainted. */
> 	if (!crc)
> 		return 1;
>+	crcval = *crc;
>
> 	/* No versions at all?  modprobe --force does this. */
> 	if (versindex == 0)
> 		return try_to_force_load(mod, symname) == 0;
>
> 	versions = (void *)sechdrs[versindex].sh_addr;
>-	num_versions = sechdrs[versindex].sh_size
>-		/ sizeof(struct modversion_info);
>+	end = (void *)versions + sechdrs[versindex].sh_size;
>
>-	for (i = 0; i < num_versions; i++) {
>-		u32 crcval;
>-
>-		if (strcmp(versions[i].name, symname) != 0)
>+	for (; versions < end; versions = (void *)versions + versions->next) {
>+		if (strcmp(versions->name, symname) != 0)
> 			continue;
>
>-		crcval = *crc;
>-		if (versions[i].crc == crcval)
>+		if (versions->crc == crcval)
> 			return 1;
>-		pr_debug("Found checksum %X vs module %lX\n",
>-			 crcval, versions[i].crc);
>+		pr_debug("Found checksum %X vs module %X\n",
>+			 crcval, versions->crc);
> 		goto bad_version;
> 	}
>
>diff --git a/scripts/export_report.pl b/scripts/export_report.pl
>index feb3d5542a62..1117646f3141 100755
>--- a/scripts/export_report.pl
>+++ b/scripts/export_report.pl
>@@ -116,18 +116,19 @@ foreach my $thismod (@allcfiles) {
> 	while ( <$module> ) {
> 		chomp;
> 		if ($state == 0) {
>-			$state = 1 if ($_ =~ /static const struct modversion_info/);
>+			$state = 1 if ($_ =~ /static const char ____versions/);
> 			next;
> 		}
> 		if ($state == 1) {
>-			$state = 2 if ($_ =~ /__attribute__\(\(section\("__versions"\)\)\)/);
>+			$state = 2 if ($_ =~ /__used __section\("__versions"\)/);
> 			next;
> 		}
> 		if ($state == 2) {
>-			if ( $_ !~ /0x[0-9a-f]+,/ ) {
>+			if ( $_ !~ /\\0"/ ) {
>+				last if ($_ =~ /;/);
> 				next;
> 			}
>-			my $sym = (split /([,"])/,)[4];
>+			my $sym = (split /(["\\])/,)[2];
> 			my ($module, $value, $symbol, $gpl) = @{$SYMBOL{$sym}};
> 			$SYMBOL{ $sym } =  [ $module, $value+1, $symbol, $gpl];
> 			push(@{$MODULE{$thismod}} , $sym);
>diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
>index efff8078e395..334d170de31f 100644
>--- a/scripts/mod/modpost.c
>+++ b/scripts/mod/modpost.c
>@@ -2046,13 +2046,17 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
> static void add_versions(struct buffer *b, struct module *mod)
> {
> 	struct symbol *s;
>+	unsigned int name_len;
>+	unsigned int name_len_padded;
>+	unsigned int tmp;
>+	unsigned char *tmp_view = (unsigned char *)&tmp;
>
> 	if (!modversions)
> 		return;
>
> 	buf_printf(b, "\n");
>-	buf_printf(b, "static const struct modversion_info ____versions[]\n");
>-	buf_printf(b, "__used __section(\"__versions\") = {\n");
>+	buf_printf(b, "static const char ____versions[]\n");
>+	buf_printf(b, "__used __section(\"__versions\") =\n");
>
> 	list_for_each_entry(s, &mod->unresolved_symbols, list) {
> 		if (!s->module)
>@@ -2062,16 +2066,25 @@ static void add_versions(struct buffer *b, struct module *mod)
> 				s->name, mod->name);
> 			continue;
> 		}
>-		if (strlen(s->name) >= MODULE_NAME_LEN) {
>-			error("too long symbol \"%s\" [%s.ko]\n",
>-			      s->name, mod->name);
>-			break;
>-		}
>-		buf_printf(b, "\t{ %#8x, \"%s\" },\n",
>-			   s->crc, s->name);
>+		name_len = strlen(s->name);
>+		name_len_padded = (name_len + 1 + 3) & ~3;
>+
>+		/* Offset to next entry */
>+		tmp = TO_NATIVE(8 + name_len_padded);
>+		buf_printf(b, "\t\"\\x%02x\\x%02x\\x%02x\\x%02x",
>+			   tmp_view[0], tmp_view[1], tmp_view[2], tmp_view[3]);
>+
>+		tmp = TO_NATIVE(s->crc);
>+		buf_printf(b, "\\x%02x\\x%02x\\x%02x\\x%02x\"\n",
>+			   tmp_view[0], tmp_view[1], tmp_view[2], tmp_view[3]);
>+
>+		buf_printf(b, "\t\"%s", s->name);
>+		for (; name_len < name_len_padded; name_len++)
>+			buf_printf(b, "\\0");
>+		buf_printf(b, "\"\n");
> 	}
>
>-	buf_printf(b, "};\n");
>+	buf_printf(b, ";\n");
> }
>
> static void add_depends(struct buffer *b, struct module *mod)
>-- 
>2.34.1
>