[syslinux] Acceptable version mismatch between syslinux 6.0N's MBR/ldlinux.sys and *.c32?

[intrigeri]

Hi there,

first, thanks a lot for syslinux!

I'm aware that one can't mix syslinux 4's MBR + ldlinux.sys with
syslinux 6's COM32R modules. Fair enough. Now, I need to know how
strong this "versions *must* match" requirement is when dealing with
different versions of syslinux 6.x. E.g.

  * MBR and ldlinux.sys installed by syslinux 6.03-pre1
  * all *.c32 modules installed by 6.03-pre14

Has this a chance to work?

Of course, feel free to ask me to just try myself. Still, assuming
this currently works fine, it would be good to know to what extent
such incompatibilities are likely to happen in the future, when mixing
bits of different versions of the 6.x branch.

Can anyone make a rough guess?

Context: Tails [1] has an "Upgrade from ISO" feature, that allows
upgrading USB stick B, while running from USB stick A, using
a user-provided ISO image. Generally, this is used to upgrade stick
B from Tails version N to N+1, while stick A is still running version
N. (If stick A was running version N+1, the user could simply use our
"Clone and Upgrade" feature instead). During this process, the MBR and
ldlinux.sys are upgraded, using the version of syslinux shipped in
stick A. Generally speaking, when Tails version N+1 ships a newer
version of syslinux than the one shipped in Tails N, we're then going
to mix an older syslinux MBR + ldlinux.sys, with newer COM32R modules.
This can't possibly work for the syslinux 4 -> 6 upgrade, but we'll
work around that problem somehow, UX will be degraded during this
upgrade, but oh well. My question is about future upgrades between
point-releases of syslinux 6. So, my question really is: once all
Tails users have a stick in version N that ships, say, syslinux
6.03-pre1, and the next upgrade to version N ships with 6.03-pre14's
COM32R modules, will running syslinux 6.03-pre1 on the upgraded stick
(that has 6.03-pre14 COM32R modules) make it unbootable?

[1] https://tails.boum.org/

Cheers,
-- 
  intrigeri
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