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pauth: Create new feature string for pauth to prevent crashing older gdb's

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Older gdb's (9, 10, 11 and 12) have a bug that causes them to crash whenever
a target reports the pauth feature string in the target description and also
provide additional register outside of gdb's known and expected feature
strings.

This was fixed in gdb 13 onwards, but that means we're stuck with gdb's out
there that will crash on connection to the above targets.

QEMU has postponed inclusion of the pauth feature string in version 8, and
instead we agreed to use a new feature name to prevent crashing those older
gdb's.

Initially there was a plan to backport a trivial fix all the way to gdb 9, but
given QEMU's choice, this is no longer needed.

This new feature string is org.gnu.gdb.aarch64.pauth_v2, and should be used
by all targets going forward, except native linux gdb and gdbserver, for
backwards compatibility with older gdb's/gdbserver's.

gdb/gdbserver will still emit the old feature string for Linux since it doesn't
report additional system registers and thus doesn't cause a crash of older
gdb's. We can revisit this in the future once the problematic gdb's are likely
no longer in use.

I've added some documentation to explain the situation.
This commit is contained in:
Luis Machado 2023-04-03 10:43:34 +01:00
parent e10d82fc3e
commit acdf60711d
3 changed files with 41 additions and 1 deletions
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4
gdb/NEWS
View file

@ -3,6 +3,10 @@
*** Changes since GDB 13
* The AArch64 'org.gnu.gdb.aarch64.pauth' Pointer Authentication feature string
has been deprecated in favor of the 'org.gnu.gdb.aarch64.pauth_v2' feature
string.
* GDB now has some support for integer types larger than 64 bits.
* Removed targets and native configurations

15
gdb/aarch64-tdep.c
View file

@ -3500,8 +3500,15 @@ aarch64_features_from_target_desc (const struct target_desc *tdesc)
return features;
features.vq = aarch64_get_tdesc_vq (tdesc);
/* We need to look for a couple pauth feature name variations. */
features.pauth
= (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.pauth") != nullptr);
if (!features.pauth)
features.pauth = (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.pauth_v2")
!= nullptr);
features.mte
= (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.mte") != nullptr);
@ -3679,7 +3686,6 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
feature_core = tdesc_find_feature (tdesc,"org.gnu.gdb.aarch64.core");
feature_fpu = tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.fpu");
feature_sve = tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.sve");
feature_pauth = tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.pauth");
const struct tdesc_feature *feature_mte
= tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.mte");
const struct tdesc_feature *feature_tls
@ -3773,6 +3779,13 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
}
}
/* We have two versions of the pauth target description due to a past bug
where GDB would crash when seeing the first version of the pauth target
description. */
feature_pauth = tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.pauth");
if (feature_pauth == nullptr)
feature_pauth = tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.pauth_v2");
/* Add the pauth registers. */
int pauth_masks = 0;
if (feature_pauth != NULL)

23
gdb/doc/gdb.texinfo
View file

@ -47991,6 +47991,29 @@ has a signed link register value that needs to be unmasked/decoded.
Extra registers are allowed in this feature, but they will not affect
@value{GDBN}.
Please note the @samp{org.gnu.gdb.aarch64.pauth} feature string is deprecated
and must only be used for backwards compatibility with older releases of
@value{GDBN} and @command{gdbserver}. Targets that support Pointer
Authentication must advertise such capability by using the
@samp{org.gnu.gdb.aarch64.pauth_v2} feature string instead.
The @samp{org.gnu.gdb.aarch64.pauth_v2} feature has the exact same contents
as feature @samp{org.gnu.gdb.aarch64.pauth}.
The reason for having feature @samp{org.gnu.gdb.aarch64.pauth_v2} is a bug in
previous versions of @value{GDBN} (versions 9, 10, 11 and 12). This bug
caused @value{GDBN} to crash whenever the target reported support for Pointer
Authentication (using feature string @samp{org.gnu.gdb.aarch64.pauth}) and also
reported additional system registers that were not accounted for by
@value{GDBN}. This is more common when using emulators and on bare-metal
debugging scenarios.
It can also happen if a newer gdbserver is used with an old @value{GDBN} that
has the bug. In such a case, the newer gdbserver might report Pointer
Authentication support via the @samp{org.gnu.gdb.aarch64.pauth} feature string
and also report additional registers the older @value{GDBN} does not know
about, potentially leading to a crash.
@subsubsection AArch64 TLS registers feature
The @samp{org.gnu.gdb.aarch64.tls} optional feature was introduced to expose