Hi Michael,
On Fri, Mar 29, 2024 at 01:41:37PM +0100, Michael Weiß wrote:
> finit_module() supports the MODULE_INIT_COMPRESS_FILE flag since
> Linux 5.17. See commit b1ae6dc41eaaa ("module: add in-kernel support
> for decompressing")
>
> During implementation of a secure module loader in GyroidOS, we
> wanted to filter unsafe module parameters. To verify that only the
> two documented flags which are disabling sanity checks are unsafe,
> we had a look in the current kernel implementation.
>
> We discovered that this new flag MODULE_INIT_COMPRESS_FILE was added.
> Having a deeper look at the code, we also discovered that a new error
> code EOPNOTSUPP is possible within newer kernels.
>
> The inital commit only supported gzip and xz compression algorithms.
> Support for zstd was added in Linux 6.2 by commit 169a58ad824d8
> ("module/decompress: Support zstd in-kernel decompression")
>
> Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
> ---
> man2/init_module.2 | 48 +++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/man2/init_module.2 b/man2/init_module.2
> index 95917a079..8197b0df6 100644
> --- a/man2/init_module.2
> +++ b/man2/init_module.2
> @@ -107,6 +107,10 @@ Ignore symbol version hashes.
> .TP
> .B MODULE_INIT_IGNORE_VERMAGIC
> Ignore kernel version magic.
> +.TP
> +.BR MODULE_INIT_COMPRESSED_FILE " (since Linux 5.17)"
> +.\" commit b1ae6dc41eaaa98bb75671e0f3665bfda248c3e7
> +Use in-kernel module decompression.
> .P
> There are some safety checks built into a module to ensure that
> it matches the kernel against which it is loaded.
> @@ -136,6 +140,41 @@ If the kernel is built to permit forced loading (i.e., configured with
> then loading continues, otherwise it fails with the error
> .B ENOEXEC
> as expected for malformed modules.
> +.P
> +If the kernel was build with
> +.BR CONFIG_MODULE_DECOMPRESS ,
> +the in-kernel decompression feature can be used.
> +Userspace code can check if the kernel supports decompression by
> +reading the
> +.I /sys/module/compression
> +attribute.
> +If the kernel supports decompression, the compressed file can directly
Please use semantic newlines. See man-pages(7):
$ MANWIDTH=72 man man-pages | sed -n '/Use semantic newlines/,/^$/p';
Use semantic newlines
In the source of a manual page, new sentences should be started
on new lines, long sentences should be split into lines at clause
breaks (commas, semicolons, colons, and so on), and long clauses
should be split at phrase boundaries. This convention, sometimes
known as "semantic newlines", makes it easier to see the effect
of patches, which often operate at the level of individual sen‐
tences, clauses, or phrases.
> +be passed to
> +.BR finit_module ()
> +using the
> +.B MODULE_INIT_COMPRESSED_FILE
> +flag.
> +The in-kernel module decompressor supports the following compression
> +algorithms:
> +.P
> +.RS 4
> +.PD 0
> +.IP \[bu] 3
> +.I gzip
> +(since Linux 5.17)
> +.IP \[bu]
> +.I xz
> +(since Linux 5.17)
> +.IP \[bu]
> +.I zstd
> +.\" commit 169a58ad824d896b9e291a27193342616e651b82
> +(since Linux 6.2)
> +.PD
> +.RE
> +.P
> +The kernel only implements a single decompression method which is
> +selected during module generation accordingly to the compression
> +method selected in the kernel configuration.
> .SH RETURN VALUE
> On success, these system calls return 0.
> On error, \-1 is returned and
> @@ -221,12 +260,19 @@ is too large.
> .TP
> .B EINVAL
> .I flags
> -is invalid.
> +is invalid or the decompressor sanity checks failed while loading
> +a compressed module with flag
> +.BR CONFIG_MODULE_DECOMPRESS
This should use B, not BR. (It uses Bold, not Bold/Roman alternating.)
> +set.
> .TP
> .B ENOEXEC
> .I fd
> does not refer to an open file.
> .TP
> +.BR EOPNOTSUPP " (since Linux 5.17)"
> +This error is return if the kernel was configured without
The first words seems redundant. I'd use:
The kernel was configured without CONFIG_MODULE_DECOMPRESS.
Which seems incomplete. I guess if the module is not compressed, then
it won't report this error.
The module is compressed, and the kernel was built without ...
> +.BR CONFIG_MODULE_DECOMPRESS
This was missing a terminating '.'.
> +.TP
> .BR ETXTBSY " (since Linux 4.7)"
> .\" commit 39d637af5aa7577f655c58b9e55587566c63a0af
> The file referred to by
> --
> 2.39.2
>
Have a lovely night!
Alex
--
<https://www.alejandro-colomar.es/>
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