[RFC] efi: variable name size limit in efivar_init()

[RFC] efi: variable name size limit in efivar_init()

[Heinrich Schuchardt]

Hello Ard,

in efivar_init() I read:

         /*
          * Per EFI spec, the maximum storage allocated for both
          * the variable name and variable data is 1024 bytes.
          */

         do {
                 variable_name_size = 1024;

This dates back at least to Linux 2.5.0 of 23-Nov-2001 where in
arch/ia64/kernel/efivars.c we have:

/*
  * The maximum size of VariableName + Data = 1024
  * Therefore, it's reasonable to save that much
  * space in each part of the structure,
  * and we use a page for reading/writing.
  */

I cannot find this 1024 byte size limit neither in UEFI spec 2.8B nor in
EFI spec 1.1.

Variable d719b2cb-3d3a-4596-a3bc-dad00e67656f-db has 5080 bytes of data
on my laptop.

It may not reasonable to use a variable name exceeding 512 UTF-16
characters. But shouldn't we handle this case gracefully in efivar_init()?

Best regards

Heinrich

Re: [RFC] efi: variable name size limit in efivar_init()

[Ard Biesheuvel]

(cc Matthew and Peter)

On Wed, 17 Mar 2021 at 08:35, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
>
>
> Hello Ard,
>
> in efivar_init() I read:
>
>          /*
>           * Per EFI spec, the maximum storage allocated for both
>           * the variable name and variable data is 1024 bytes.
>           */
>
>          do {
>                  variable_name_size = 1024;
>
> This dates back at least to Linux 2.5.0 of 23-Nov-2001 where in
> arch/ia64/kernel/efivars.c we have:
>
> /*
>   * The maximum size of VariableName + Data = 1024
>   * Therefore, it's reasonable to save that much
>   * space in each part of the structure,
>   * and we use a page for reading/writing.
>   */
>
> I cannot find this 1024 byte size limit neither in UEFI spec 2.8B nor in
> EFI spec 1.1.
>
> Variable d719b2cb-3d3a-4596-a3bc-dad00e67656f-db has 5080 bytes of data
> on my laptop.
>
> It may not reasonable to use a variable name exceeding 512 UTF-16
> characters. But shouldn't we handle this case gracefully in efivar_init()?
>

This predates my involvement in EFI by a long time, so I've cc'ed
Peter and Matthew, who may be able to shed some light on this.

Re: [RFC] efi: variable name size limit in efivar_init()

[Ard Biesheuvel]

On Wed, 17 Mar 2021 at 14:32, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> (cc Matthew and Peter)
>

Now for real... (thanks Heinrich)

> On Wed, 17 Mar 2021 at 08:35, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> >
> >
> > Hello Ard,
> >
> > in efivar_init() I read:
> >
> >          /*
> >           * Per EFI spec, the maximum storage allocated for both
> >           * the variable name and variable data is 1024 bytes.
> >           */
> >
> >          do {
> >                  variable_name_size = 1024;
> >
> > This dates back at least to Linux 2.5.0 of 23-Nov-2001 where in
> > arch/ia64/kernel/efivars.c we have:
> >
> > /*
> >   * The maximum size of VariableName + Data = 1024
> >   * Therefore, it's reasonable to save that much
> >   * space in each part of the structure,
> >   * and we use a page for reading/writing.
> >   */
> >
> > I cannot find this 1024 byte size limit neither in UEFI spec 2.8B nor in
> > EFI spec 1.1.
> >
> > Variable d719b2cb-3d3a-4596-a3bc-dad00e67656f-db has 5080 bytes of data
> > on my laptop.
> >
> > It may not reasonable to use a variable name exceeding 512 UTF-16
> > characters. But shouldn't we handle this case gracefully in efivar_init()?
> >
>
> This predates my involvement in EFI by a long time, so I've cc'ed
> Peter and Matthew, who may be able to shed some light on this.