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fix(native): a SysV segment stays until IPC_RMID even after its maker ends

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Kurt Himebauch committed 2026-11-09 23:11:36 -05:00
1 parent 5831fc9a57
commit 517f48895b
1 file changed
+6 -48
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tools/nativeguest/hook/sysv_shm.c
+6 -48
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@@ -15,8 +15,7 @@
// N is chosen at random and claimed with O_EXCL, so processes never pick the same one. IPC_RMID
// removes the files at once: programs that attached the segment keep their mapping (as on Linux),
// later attaches fail, which is what X programs expect (they remove a segment as soon as the X
// server has attached it). A private segment whose maker and every process that attached it have
// ended without removing it is deleted by the next shmget() that creates one; a keyed one stays.
// server has attached it). A segment nobody removed stays until the session's directory is cleared.
#ifndef DDNATV_HOST_TEST
@@ -154,10 +153,8 @@ static void shm_unlock_dir(int fd) {
if (fd >= 0) syscall(SYS_close, fd);
}
// Deletes the private (IPC_PRIVATE) segments whose maker and attached processes have all ended (a
// segment being written has no description yet and is left alone), and the marks of attached
// processes that have ended. A segment with a key stays until IPC_RMID, as on Linux: another
// process can still look it up by its key.
// Deletes the marks of attached processes that have ended. Every segment stays until IPC_RMID, as on
// Linux: a private one's maker can hand its id to a process that attaches after the maker has ended.
static void shm_collect(void) {
char path[PATH_MAX];
if (shm_path(path, sizeof(path), NULL) != 0) return;
@@ -169,8 +166,6 @@ static void shm_collect(void) {
return;
}
char dents[4096];
int dead[64];
int ndead = 0;
for (;;) {
long n = syscall(SYS_getdents64, dfd, dents, sizeof(dents));
if (n <= 0) break;
@@ -179,53 +174,16 @@ static void shm_collect(void) {
memcpy(&reclen, dents + at + 16, sizeof(reclen));
if (reclen == 0) break;
const char *entry = dents + at + 19;
if (strncmp(entry, "id-", 3) == 0 && ndead < 64) {
int id = atoi(entry + 3);
char text[128];
struct shm_meta meta;
if (shm_read_small(entry, text, sizeof(text)) > 0 &&
sscanf(text, "%d %lx %o", &meta.pid, &meta.key, &meta.mode) == 3 && meta.key == IPC_PRIVATE &&
!shm_process_alive(meta.pid)) {
dead[ndead++] = id;
}
}
int id = 0, pid = 0;
if (strncmp(entry, "att-", 4) == 0 && sscanf(entry + 4, "%d-%d", &id, &pid) == 2 && !shm_process_alive(pid))
shm_unlink_file(entry);
at += reclen;
}
}
// a segment still attached somewhere is kept, as Linux keeps it until IPC_RMID
int used[64] = { 0 };
syscall(SYS_close, dfd);
dfd = ndead > 0 ? (int)syscall(SYS_openat, AT_FDCWD, path, O_RDONLY | O_DIRECTORY | O_CLOEXEC) : -1;
if (dfd >= 0) {
for (;;) {
long n = syscall(SYS_getdents64, dfd, dents, sizeof(dents));
if (n <= 0) break;
for (long at = 0; at < n;) {
unsigned short reclen;
memcpy(&reclen, dents + at + 16, sizeof(reclen));
if (reclen == 0) break;
const char *entry = dents + at + 19;
int id = 0, pid = 0;
if (strncmp(entry, "att-", 4) == 0 && sscanf(entry + 4, "%d-%d", &id, &pid) == 2) {
if (shm_process_alive(pid)) {
for (int i = 0; i < ndead; i++) if (dead[i] == id) used[i] = 1;
} else {
shm_unlink_file(entry);
}
}
at += reclen;
}
}
}
if (dfd >= 0) syscall(SYS_close, dfd);
for (int i = 0; i < ndead; i++) {
struct shm_meta meta;
if (!used[i] && shm_read_meta(dead[i], &meta) == 0) shm_remove(dead[i], meta.key);
}
shm_unlock_dir(lock);
}
// A new segment: claims an id, makes its memory, then writes its description. Returns the id or -1.
static int shm_create(long key, size_t size, unsigned mode) {
// a different sequence of ids for every call, also within one second and one process
static unsigned calls = 0;