[RFC 02/14] efi_loader: library function efi_dp_merge

[Ilias Apalodimas]

Hi Heinrich

On Fri, 26 Apr 2024 at 17:53, Heinrich Schuchardt
<heinrich.schuchardt at canonical.com> wrote:
>
> On 26.04.24 16:30, Ilias Apalodimas wrote:
> > Hi Heinrich,
> >
> > On Fri, 26 Apr 2024 at 17:13, Heinrich Schuchardt
> > <heinrich.schuchardt at canonical.com> wrote:
> >>
> >> Provide a function to append a device_path to a list of device paths
> >> that is separated by final end nodes.
> >>
> >> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> >> ---
> >>   include/efi_loader.h             |  3 +++
> >>   lib/efi_loader/efi_device_path.c | 31 +++++++++++++++++++++++++++++++
> >>   2 files changed, 34 insertions(+)
> >>
> >> diff --git a/include/efi_loader.h b/include/efi_loader.h
> >> index 9600941aa32..a7d7b8324f1 100644
> >> --- a/include/efi_loader.h
> >> +++ b/include/efi_loader.h
> >> @@ -944,6 +944,9 @@ struct efi_load_option {
> >>
> >>   struct efi_device_path *efi_dp_from_lo(struct efi_load_option *lo,
> >>                                         const efi_guid_t *guid);
> >> +struct efi_device_path *efi_dp_merge(const struct efi_device_path *dp1,
> >> +                                    efi_uintn_t *size,
> >> +                                    const struct efi_device_path *dp2);
> >>   struct efi_device_path *efi_dp_concat(const struct efi_device_path *dp1,
> >>                                        const struct efi_device_path *dp2,
> >>                                        bool split_end_node);
> >> diff --git a/lib/efi_loader/efi_device_path.c b/lib/efi_loader/efi_device_path.c
> >> index 46aa59b9e40..16cbe41d32f 100644
> >> --- a/lib/efi_loader/efi_device_path.c
> >> +++ b/lib/efi_loader/efi_device_path.c
> >> @@ -270,6 +270,37 @@ struct efi_device_path *efi_dp_dup(const struct efi_device_path *dp)
> >>          return ndp;
> >>   }
> >>
> >> +/**
> >> + * efi_dp_merge() - Concatenate two device paths separated by final end node
> >> + *
> >> + * @dp1:       first device path
> >> + * @size:      pointer to length of @dp1, total size on return
> >> + * @dp2:       second device path
> >> + *
> >> + * Return:     concatenated device path or NULL
> >> + */
> >> +struct efi_device_path *efi_dp_merge(const struct efi_device_path *dp1,
> >> +                                    efi_uintn_t *size,
> >> +                                    const struct efi_device_path *dp2)
> >> +{
> >> +       efi_uintn_t len, len1, len2;
> >> +       struct efi_device_path *dp;
> >> +       u8 *p;
> >> +
> >> +       len1 = *size;
> >> +       len2 = efi_dp_size(dp2) + sizeof(END);
> >> +       len = len1 + len2;
> >> +       dp = efi_alloc(len);
> >> +       if (!dp)
> >> +               return NULL;
> >> +       memcpy(dp, dp1, len1);
> >> +       p = (u8 *)dp + len1;
> >> +       memcpy(p, dp2, len2);
> >> +       *size = len;
> >
> > Can't we just use efi_dp_concat()?
>
> efi_dp_concat cannot be used to put a device-path end-node in between
> two device-paths that are concatenated. We need that separator in the
> load options.

I am not sure I am following you. It's been a few years so please bear
with me until I manage to re-read that code and page it back to
memory.

What I remember though is that the format of the DP looks like this:

Loaded image DP - end node - initrd GUID DP (without and end node) -
initrd - end of device path.
So to jump from the special initrd GUID to the actual DP that contains
the initrd you need to do an efi_dp_next(). Also, efi_dp_concat() will
inject an end node with a DEVICE_PATH_SUB_TYPE_END type if the third
argument is 'true'.

What am I missing?

Thanks
/Ilias

>
> Best regards
>
> Heinrich
>
> >
> > Thanks
> > /Ilias
> >> +
> >> +       return dp;
> >> +}
> >> +
> >>   /**
> >>    * efi_dp_concat() - Concatenate two device paths and add and terminate them
> >>    *                   with an end node.
> >> --
> >> 2.43.0
> >>
>