Re: [Libguestfs] [PATCH libnbd] generator: Pass LISTEN_FDNAMES=nbd with systemd socket activation

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.

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

(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.

(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 :)

(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...;. snprintf() and sprintf() are not on that list, so it makes sense for nbd_internal_fork_safe_itoa() to exist.

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.

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?

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