Re: [PATCH] modpost: move the namespace field in Module.symvers last

[Jessica Yu <jeyu@kernel.org>]

+++ Matthias Maennich [05/03/20 08:59 +0000]:
>Hi Jessica!
>Thanks for working on this!
>
>On Wed, Mar 04, 2020 at 06:03:45PM +0100, Jessica Yu wrote:
>>In order to preserve backwards compatability with kmod tools, we have to
>>move the namespace field in Module.symvers last, as the depmod -e -E
>>option looks at the first three fields in Module.symvers to check symbol
>>versions (and it's expected they stay in the original order of crc,
>>symbol, module).
>>
>>Fixes: cb9b55d21fe0 ("modpost: add support for symbol namespaces")
>>Cc: stable@vger.kernel.org
>
>Please note, this patch did not actually go to stable@.

Hi Matthias! Thanks, I missed that.

>>Signed-off-by: Jessica Yu <jeyu@kernel.org>
>>---
>>Documentation/kbuild/modules.rst |  4 ++--
>>scripts/export_report.pl         |  2 +-
>>scripts/mod/modpost.c            | 24 ++++++++++++------------
>>3 files changed, 15 insertions(+), 15 deletions(-)
>>
>>diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst
>>index 69fa48ee93d6..e0b45a257f21 100644
>>--- a/Documentation/kbuild/modules.rst
>>+++ b/Documentation/kbuild/modules.rst
>>@@ -470,9 +470,9 @@ build.
>>
>>	The syntax of the Module.symvers file is::
>>
>>-	<CRC>       <Symbol>          <Namespace>  <Module>                         <Export Type>
>>+	<CRC>       <Symbol>         <Module>                         <Export Type>     <Namespace>
>>
>>-	0xe1cc2a05  usb_stor_suspend  USB_STORAGE  drivers/usb/storage/usb-storage  EXPORT_SYMBOL_GPL
>>+	0xe1cc2a05  usb_stor_suspend drivers/usb/storage/usb-storage  EXPORT_SYMBOL_GPL USB_STORAGE
>>
>>	The fields are separated by tabs and values may be empty (e.g.
>>	if no namespace is defined for an exported symbol).
>>diff --git a/scripts/export_report.pl b/scripts/export_report.pl
>>index 548330e8c4e7..feb3d5542a62 100755
>>--- a/scripts/export_report.pl
>>+++ b/scripts/export_report.pl
>>@@ -94,7 +94,7 @@ if (defined $opt{'o'}) {
>>#
>>while ( <$module_symvers> ) {
>>	chomp;
>>-	my (undef, $symbol, $namespace, $module, $gpl) = split('\t');
>>+	my (undef, $symbol, $module, $gpl, $namespace) = split('\t');
>>	$SYMBOL { $symbol } =  [ $module , "0" , $symbol, $gpl];
>>}
>>close($module_symvers);
>>diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
>>index 7edfdb2f4497..6ab235354f36 100644
>>--- a/scripts/mod/modpost.c
>>+++ b/scripts/mod/modpost.c
>>@@ -2427,7 +2427,7 @@ static void write_if_changed(struct buffer *b, const char *fname)
>>}
>>
>>/* parse Module.symvers file. line format:
>>- * 0x12345678<tab>symbol<tab>module[[<tab>export]<tab>something]
>>+ * 0x12345678<tab>symbol<tab>module<tab>export<tab>namespace
>> **/
>>static void read_dump(const char *fname, unsigned int kernel)
>>{
>>@@ -2440,7 +2440,7 @@ static void read_dump(const char *fname, unsigned int kernel)
>>		return;
>>
>>	while ((line = get_next_line(&pos, file, size))) {
>>-		char *symname, *namespace, *modname, *d, *export, *end;
>>+		char *symname, *namespace, *modname, *d, *export;
>>		unsigned int crc;
>>		struct module *mod;
>>		struct symbol *s;
>>@@ -2448,16 +2448,16 @@ static void read_dump(const char *fname, unsigned int kernel)
>>		if (!(symname = strchr(line, '\t')))
>>			goto fail;
>>		*symname++ = '\0';
>>-		if (!(namespace = strchr(symname, '\t')))
>>-			goto fail;
>>-		*namespace++ = '\0';
>>-		if (!(modname = strchr(namespace, '\t')))
>>+		if (!(modname = strchr(symname, '\t')))
>>			goto fail;
>>		*modname++ = '\0';
>>-		if ((export = strchr(modname, '\t')) != NULL)
>>-			*export++ = '\0';
>>-		if (export && ((end = strchr(export, '\t')) != NULL))
>>-			*end = '\0';
>>+		if (!(export = strchr(modname, '\t')))
>>+			goto fail;
>>+		*export++ = '\0';
>>+		if (!(namespace = strchr(export, '\t')))
>
>As mentioned below, we should probably treat namespace as an optional
>field. Then this needs adjusting to handle that case. Similar to how
>optional cases were handled before.

Hm, I think introducing optional fields would add unnecessary
complexity, and make future parsing harder. For example, say in the
distant future we add another field. If fields are optional, we are no
longer able to tell if the 4th field is a namespace or the new_field.
Whereas, if we made the fields mandatory (even if empty), we would see
crc<tab>symbol<tab>module<tab>export_type<tab><tab>new_field, and it's
clear that the namespace is empty. I hope that makes sense...

IMO, I think it's easiest to just establish the fact that
Module.symvers has 5 fields, and fields can be empty. If a field an
empty, then the next delimiter or end of line will just follow
immediately.

Just to reiterate, it is true namespaces are optional, and in the case
of no namespace, I would prefer it to be an empty string/field rather
than omitting it entirely.

>>+			goto fail;
>>+		*namespace++ = '\0';
>>+
>>		crc = strtoul(line, &d, 16);
>>		if (*symname == '\0' || *modname == '\0' || *d != '\0')
>>			goto fail;
>>@@ -2508,9 +2508,9 @@ static void write_dump(const char *fname)
>>				namespace = symbol->namespace;
>>				buf_printf(&buf, "0x%08x\t%s\t%s\t%s\t%s\n",
>
>This creates trailing tabs for symbols without namespace. If we treat
>'namespace' as an optional field, we should probably make the tab
>conditional as well? What do you think?

Yeah you're right, the trailing tab after an empty namespace looks
weird...but for reasons I cited above, I would like to keep it simple,
unless there are huge objections.

Thank you for the review!

Jessica