Serious compatibility breakage in -current.

Carl Shapiro carl.shapiro at gmail.com
Thu Nov 29 02:50:54 PST 2007


Developers,

FreeBSD 7 has changed the protection violation signal from SIGBUS to
SIGSEGV.  Unfortunately, when an old binary is run on a current
FreeBSD system, protection violations cause a SIGSEGV to be delivered
to the executable instead of the anticipated SIGBUS.  Binaries
compiled on older versions of FreeBSD are not prepared to handle
SIGSEGV in response to page protection violations.

One consequence of this change is that applications that handle page
protection violations, such as the CMUCL Lisp compiler have broken.
CMUCL binaries compiled for FreeBSD 4, FreeBSD 5, and FreeBSD 6 crash
on -current.  Investments FreeBSD users have in binaries that handle
page protection violations, such as things that link against boehm-gc,
are lost forever.  This significantly increases the cost of an upgrade
to FreeBSD 7.

The switch from SIGBUS to SIGSEGV is well motivated.  However, it is
not clear that consideration was given to binaries running under
compat{4,5,6}x.  One would expect the compat infrastructure to
translate protection violation signals from SIGSEGV to SIGBUS while
executing older binaries.  This would ensure that the applications
that establish SIGBUS handlers to catch protection violations continue
to work.

Among the open source operating systems the FreeBSD project has one of
the best traditions of supporting old binaries.  It would truly be a
shame for this tradition to abruptly end as of the next major release.
 As a developer who supports several versions of FreeBSD through
binary compatibility anything that can help us avoid breakage as of
the next FreeBSD release would be appreciated.

Thanks,

Carl (and the many FreeBSD users of CMUCL)


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