Richard W.M. Jones
2023-Jan-31 13:07 UTC
[Libguestfs] [PATCH libnbd] generator: Pass LISTEN_FDNAMES=nbd with systemd socket activation
On Tue, Jan 31, 2023 at 01:49:53PM +0100, Laszlo Ersek wrote:> On 1/28/23 13:47, Richard W.M. Jones wrote: > > systemd allows sockets passed through socket activation to be named > > with the protocol they require. We only ever pass one socket, name > > it. This environment variable is currently ignored by qemu-nbd and > > nbdkit, but might be used by qemu-storage-daemon: > > > > https://lists.nongnu.org/archive/html/qemu-devel/2023-01/msg06114.html > > --- > > generator/states-connect-socket-activation.c | 41 +++++++++++--------- > > 1 file changed, 23 insertions(+), 18 deletions(-) > > > > diff --git a/generator/states-connect-socket-activation.c b/generator/states-connect-socket-activation.c > > index 9a83834915..22f06d4fd3 100644 > > --- a/generator/states-connect-socket-activation.c > > +++ b/generator/states-connect-socket-activation.c > > @@ -34,16 +34,18 @@ > > /* This is baked into the systemd socket activation API. */ > > #define FIRST_SOCKET_ACTIVATION_FD 3 > > > > -/* == strlen ("LISTEN_PID=") | strlen ("LISTEN_FDS=") */ > > -#define PREFIX_LENGTH 11 > > - > > extern char **environ; > > > > /* Prepare environment for calling execvp when doing systemd socket > > * activation. Takes the current environment and copies it. Removes > > - * any existing LISTEN_PID or LISTEN_FDS and replaces them with new > > - * variables. env[0] is "LISTEN_PID=..." which is filled in by > > - * CONNECT_SA.START, and env[1] is "LISTEN_FDS=1". > > + * any existing LISTEN_PID, LISTEN_FDS or LISTEN_FDNAMES, and replaces > > + * them with new variables. > > + * > > + * env[0] is "LISTEN_PID=..." which is filled in by CONNECT_SA.START > > + * > > + * env[1] is "LISTEN_FDS=1" > > + * > > + * env[2] is "LISTEN_FDNAMES=nbd" > > */ > > static int > > prepare_socket_activation_environment (string_vector *env) > > @@ -53,26 +55,29 @@ prepare_socket_activation_environment (string_vector *env) > > > > assert (env->len == 0); > > > > - /* Reserve slots env[0] and env[1]. */ > > + /* Reserve slots env[0]..env[2] */ > > + if (string_vector_reserve (env, 3) == -1) > > + goto err; > > p = strdup ("LISTEN_PID=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"); > > if (p == NULL) > > goto err; > > - if (string_vector_append (env, p) == -1) { > > - free (p); > > - goto err; > > - } > > + string_vector_append (env, p); > > p = strdup ("LISTEN_FDS=1"); > > if (p == NULL) > > goto err; > > - if (string_vector_append (env, p) == -1) { > > - free (p); > > + string_vector_append (env, p); > > + p = strdup ("LISTEN_FDNAMES=nbd"); > > + if (p == NULL) > > goto err; > > - } > > + string_vector_append (env, p); > > > > - /* Append the current environment, but remove LISTEN_PID, LISTEN_FDS. */ > > + /* Append the current environment, but remove the special > > + * environment variables. > > + */ > > for (i = 0; environ[i] != NULL; ++i) { > > - if (strncmp (environ[i], "LISTEN_PID=", PREFIX_LENGTH) != 0 && > > - strncmp (environ[i], "LISTEN_FDS=", PREFIX_LENGTH) != 0) { > > + if (strncmp (environ[i], "LISTEN_PID=", 11) != 0 && > > + strncmp (environ[i], "LISTEN_FDS=", 11) != 0 && > > + strncmp (environ[i], "LISTEN_FDNAMES=", 15) != 0) { > > char *copy = strdup (environ[i]); > > if (copy == NULL) > > goto err; > > @@ -194,7 +199,7 @@ CONNECT_SA.START: > > char buf[32]; > > const char *v > > nbd_internal_fork_safe_itoa ((long) getpid (), buf, sizeof buf); > > - strcpy (&env.ptr[0][PREFIX_LENGTH], v); > > + strcpy (&env.ptr[0][strlen ("LISTEN_FDS=")], v); > > > > /* Restore SIGPIPE back to SIG_DFL. */ > > signal (SIGPIPE, SIG_DFL); > > I really didn't want to obsess about this -- I spent like 10+ minutes on > curbing my enthusiasm! :) --, but I believe there's a semantic bug in > the patch, one that's directly related to my "hidden" thoughts. > > (1) In the last hunk, strlen() is applied to "LISTEN_FDS=". However, the > zero-index element of the env array holds > "LISTEN_PID=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx". In other words, the patch > only gets lucky because "PID" and "FDS" are both three characters long.Yup that is totally wrong :-(> Relatedly, my hidden thought was that we shouldn't use so many naked > string literals all over the code. > > May I take a stab at rewriting this? I feel that's the easiest way for > me to express what I'd propose. Basically I'd propose: > > - an enum for listing the "keys" we need, > > - a static array of structure elements, for expressing the environment > variables (name=value), *and* the prefix lengths, > > - a macro for populating an element of the array -- use "sizeof" for > grabbing the prefix lengthSure, go for it please.> (2) Pre-patch, at commit 5a02c7d2cc6a, the error handling tail of > prepare_socket_activation_environment() is less than ideal, IMO. Namely, > we have (excerpt) > > > err: > > set_error (errno, "malloc"); > > string_vector_iter (env, (void *) free); > > free (env->ptr); > > return -1; > > (2a) we free the vector's pointer field, but don't NULL it, nor do we > zero the len or cap fields. > > We should call string_vector_reset() instead.Yup.> (2b) Casting the address of the free() function to (void*) makes me > uncomfortable. It is undefined behavior by ISO C. > > Now, I seem to remember that POSIX says in various places that pointers > to functions and pointers to void have identical representation, and > also that pointers to void and pointers to structures have identical > representation. One of those locations is the dlsym() spec > <https://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html>. > The other locations elude me, unfortunately. I think at least one of > those "other" locations may be in one of the Conformance sections; Eric > will know better. > > Regardless, casting "free" to a pointer-to-object, just because > string_vector_iter() takes a (void(*)(char*)), and not a > (void(*)(void*)), is questionable style, IMO. > > I've grepped the tree for this pattern: > > git grep -E '\(void ?\*\) ?free' > > and there are eleven hits. > > Furthermore, there are *no other* _vector_iter() calls -- and not just > string_vector_iter() calls, but in general, _vector_iter() ones! -- than > these eleven. > > I think it's time we designed either a general freeing iterator API for > vector, or at least added a trivial (stop-gap) wrapper function like > this: > > > diff --git a/common/utils/string-vector.h b/common/utils/string-vector.h > > index 80d7311debfb..5221c70e3f78 100644 > > --- a/common/utils/string-vector.h > > +++ b/common/utils/string-vector.h > > @@ -39,4 +39,10 @@ > > > > DEFINE_VECTOR_TYPE(string_vector, char *); > > > > +static inline void > > +string_free (char *string) > > +{ > > + free (string); > > +} > > + > > #endif /* STRING_VECTOR_H */ > > Comments please :)Agreed.> (3) At the last hunk, the code suggests we're between fork() and exec(). > Per POSIX > <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html>, > there we can only call async-signal-safe functions: > > > the child process may only execute async-signal-safe operations until > > such time as one of the exec functions is called > > The list of async-signal-safe functions can be found at > <https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03_03>. > snprintf() and sprintf() are not on that list, so it makes sense for > nbd_internal_fork_safe_itoa() to exist.Yes, in the past we have actually hit real bugs because of this. One I recall is: https://github.com/libguestfs/libguestfs/commit/e1c9bbb3d1d5ef81490977060120dda0963eb567 They are very hard to diagnose. This one only happened when a certain glibc feature was enabled.> The remaining functions we call in this context also seem to be on the > list... except for execvp(). > > execvp() scans PATH, and is not safe to use in this concept.That's quite annoying.> I think we should call execve() instead. First, it is async-signal-safe. > Second, it could take "env.ptr" directly; I do find the "environ" > assignment a bit dubious, even if it happens to conform to POSIX. > > What image are we executing here, to begin with? Do we really depend on > PATH searching? Or do we rely on execvp() transparently launching shell > scripts?Yes we do depend on path search. It is usually called in cases such as this: https://gitlab.com/nbdkit/libnbd/-/blob/5a02c7d2cc6a201f9e5531c0c20c2f3c22b805a2/examples/open-qcow2.c#L35 I wonder if we can just ignore this one until someone complains about the bug. Rich.> > All that said, I think we can stick with this patch; the only "actual" > problem I see with it is the "LISTEN_FDS" reference in the last hunk. > > Thanks, > Laszlo-- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
Eric Blake
2023-Jan-31 17:19 UTC
[Libguestfs] [PATCH libnbd] generator: Pass LISTEN_FDNAMES=nbd with systemd socket activation
On Tue, Jan 31, 2023 at 01:07:53PM +0000, Richard W.M. Jones wrote:> > > > I really didn't want to obsess about this -- I spent like 10+ minutes on > > curbing my enthusiasm! :) --, but I believe there's a semantic bug in > > the patch, one that's directly related to my "hidden" thoughts. > > > > (1) In the last hunk, strlen() is applied to "LISTEN_FDS=". However, the > > zero-index element of the env array holds > > "LISTEN_PID=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx". In other words, the patch > > only gets lucky because "PID" and "FDS" are both three characters long. > > Yup that is totally wrong :-( > > > Relatedly, my hidden thought was that we shouldn't use so many naked > > string literals all over the code. > > > > May I take a stab at rewriting this? I feel that's the easiest way for > > me to express what I'd propose. Basically I'd propose: > > > > - an enum for listing the "keys" we need, > > > > - a static array of structure elements, for expressing the environment > > variables (name=value), *and* the prefix lengths, > > > > - a macro for populating an element of the array -- use "sizeof" for > > grabbing the prefix length > > Sure, go for it please.More structure will pay off if we have more variables to handle in the long run. For just 3 (instead of 2), it's still a toss-up in my mind if the extra structure is worth it, but I'm not opposed to seeing an attempt at a patch along these lines.> > > (2) Pre-patch, at commit 5a02c7d2cc6a, the error handling tail of > > prepare_socket_activation_environment() is less than ideal, IMO. Namely, > > we have (excerpt) > > > > > err: > > > set_error (errno, "malloc"); > > > string_vector_iter (env, (void *) free); > > > free (env->ptr); > > > return -1; > > > > (2a) we free the vector's pointer field, but don't NULL it, nor do we > > zero the len or cap fields. > > > > We should call string_vector_reset() instead. > > Yup.The lone caller doesn't utilize env after error, but I agree that we are better off not leaving this function to expose an incomplete 'env' back to the caller in case we start using this function in more places.> > > (2b) Casting the address of the free() function to (void*) makes me > > uncomfortable. It is undefined behavior by ISO C. > > > > Now, I seem to remember that POSIX says in various places that pointers > > to functions and pointers to void have identical representation, and > > also that pointers to void and pointers to structures have identical > > representation. One of those locations is the dlsym() spec > > <https://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html>. > > The other locations elude me, unfortunately. I think at least one of > > those "other" locations may be in one of the Conformance sections; Eric > > will know better.I recall it being discussed in the Austin Group that POSIX explicitly requires void* and function pointers to be cross-assignable, but only _because_ of dlsym(). After searching for variations of 'void *', 'void pointer', 'function pointer' and 'pointer to function', I couldn't locate anything besides the dlsym() section that elaborates on the requirement that a POSIX compiler must allow conversion of a function pointer into void* and back.> > > > Regardless, casting "free" to a pointer-to-object, just because > > string_vector_iter() takes a (void(*)(char*)), and not a > > (void(*)(void*)), is questionable style, IMO. > > > > I've grepped the tree for this pattern: > > > > git grep -E '\(void ?\*\) ?free' > > > > and there are eleven hits. > > > > Furthermore, there are *no other* _vector_iter() calls -- and not just > > string_vector_iter() calls, but in general, _vector_iter() ones! -- than > > these eleven. > > > > I think it's time we designed either a general freeing iterator API for > > vector, or at least added a trivial (stop-gap) wrapper function like > > this: > > > > > diff --git a/common/utils/string-vector.h b/common/utils/string-vector.h > > > index 80d7311debfb..5221c70e3f78 100644 > > > --- a/common/utils/string-vector.h > > > +++ b/common/utils/string-vector.h > > > @@ -39,4 +39,10 @@ > > > > > > DEFINE_VECTOR_TYPE(string_vector, char *); > > > > > > +static inline void > > > +string_free (char *string) > > > +{ > > > + free (string); > > > +} > > > + > > > #endif /* STRING_VECTOR_H */ > > > > Comments please :) > > Agreed.I'm also in agreement that tweaking our vector interface for the ways in which we actively use it (without having to spell out explicit (void*) casts) would be welcome.> > > (3) At the last hunk, the code suggests we're between fork() and exec(). > > Per POSIX > > <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html>, > > there we can only call async-signal-safe functions: > > > > > the child process may only execute async-signal-safe operations until > > > such time as one of the exec functions is called > > > > The list of async-signal-safe functions can be found at > > <https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03_03>. > > snprintf() and sprintf() are not on that list, so it makes sense for > > nbd_internal_fork_safe_itoa() to exist. > > Yes, in the past we have actually hit real bugs because of this. One > I recall is: > > https://github.com/libguestfs/libguestfs/commit/e1c9bbb3d1d5ef81490977060120dda0963eb567 > > They are very hard to diagnose. This one only happened when a certain > glibc feature was enabled. > > > The remaining functions we call in this context also seem to be on the > > list... except for execvp(). > > > > execvp() scans PATH, and is not safe to use in this concept. > > That's quite annoying. > > > I think we should call execve() instead. First, it is async-signal-safe. > > Second, it could take "env.ptr" directly; I do find the "environ" > > assignment a bit dubious, even if it happens to conform to POSIX. > > > > What image are we executing here, to begin with? Do we really depend on > > PATH searching? Or do we rely on execvp() transparently launching shell > > scripts? > > Yes we do depend on path search. It is usually called in cases such > as this: > > https://gitlab.com/nbdkit/libnbd/-/blob/5a02c7d2cc6a201f9e5531c0c20c2f3c22b805a2/examples/open-qcow2.c#L35 > > I wonder if we can just ignore this one until someone complains > about the bug.The alternative to relying on execvp() to scan PATH is to pre-scan PATH ourselves before fork(). I wish there were a helper function in glibc that would quickly return the absolute path that execvp() would otherwise utilize. fexecve() also comes in handy for avoiding TOCTTOU races, but it is newer and not as widely available. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org