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nss: never become IPC clients for services that are about to be started

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This is an attempt to automatically detect and avoid certain kinds of
NSS deadlocks as discussed in this thread:

https://lists.freedesktop.org/archives/systemd-devel/2018-July/040975.html
This commit is contained in:
Lennart Poettering
2018-07-04 15:37:03 +02:00
parent 3992221767
commit 2f28018cb8
2 changed files with 73 additions and 0 deletions
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29
src/nss-resolve/nss-resolve.c
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@ -91,6 +91,20 @@ static uint32_t ifindex_to_scopeid(int family, const void *a, int ifindex) {
return IN6_IS_ADDR_LINKLOCAL(&in6) ? ifindex : 0;
}
static bool avoid_deadlock(void) {
/* Check whether this lookup might have a chance of deadlocking because we are called from the service manager
* code activating systemd-resolved.service. After all, we shouldn't synchronously do lookups to
* systemd-resolved if we are required to finish before it can be started. This of course won't detect all
* possible dead locks of this kind, but it should work for the most obvious cases. */
if (geteuid() != 0) /* Ignore the env vars unless we are privileged. */
return false;
return streq_ptr(getenv("SYSTEMD_ACTIVATION_UNIT"), "systemd-resolved.service") &&
streq_ptr(getenv("SYSTEMD_ACTIVATION_SCOPE"), "system");
}
enum nss_status _nss_resolve_gethostbyname4_r(
const char *name,
struct gaih_addrtuple **pat,
@ -116,6 +130,11 @@ enum nss_status _nss_resolve_gethostbyname4_r(
assert(errnop);
assert(h_errnop);
if (avoid_deadlock()) {
r = -EDEADLK;
goto fail;
}
r = sd_bus_open_system(&bus);
if (r < 0)
goto fail;
@ -294,6 +313,11 @@ enum nss_status _nss_resolve_gethostbyname3_r(
goto fail;
}
if (avoid_deadlock()) {
r = -EDEADLK;
goto fail;
}
r = sd_bus_open_system(&bus);
if (r < 0)
goto fail;
@ -484,6 +508,11 @@ enum nss_status _nss_resolve_gethostbyaddr2_r(
return NSS_STATUS_UNAVAIL;
}
if (avoid_deadlock()) {
r = -EDEADLK;
goto fail;
}
r = sd_bus_open_system(&bus);
if (r < 0)
goto fail;