CRASH at auto-restart syscall handling for SYSENTER

The following app causes drrun to run into a segmentation fault, possibly due to

> a bug in auto-restart syscall handling when SYSENTER is the gateway with some recent kernel changes. (cit. @derekbruening)

```
/* server.c */

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

#define PORT "3490" // the port users will be connecting to

#define BACKLOG 10 // how many pending connections queue will hold

void sigchld_handler(int s)
{
(void)s; // quiet unused variable warning

// waitpid() might overwrite errno, so we save and restore it:
int saved_errno = errno;

while(waitpid(-1, NULL, WNOHANG) > 0);

errno = saved_errno;
}

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) {
return &(((struct sockaddr_in*)sa)->sin_addr);
}

return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(void)
{
int sockfd, new_fd; // listen on sock_fd, new connection on new_fd
struct addrinfo hints, *servinfo, *p;
struct sockaddr_storage their_addr; // connector's address information
socklen_t sin_size;
struct sigaction sa;
int yes=1;
char s[INET6_ADDRSTRLEN];
int rv;

memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // use my IP

if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
return 1;
}

// loop through all the results and bind to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror("server: socket");
continue;
}

if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes,
sizeof(int)) == -1) {
perror("setsockopt");
exit(1);
}

if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror("server: bind");
continue;
}

break;
}

freeaddrinfo(servinfo); // all done with this structure

if (p == NULL) {
fprintf(stderr, "server: failed to bind\n");
exit(1);
}

if (listen(sockfd, BACKLOG) == -1) {
perror("listen");
exit(1);
}

sa.sa_handler = sigchld_handler; // reap all dead processes
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
if (sigaction(SIGCHLD, &sa, NULL) == -1) {
perror("sigaction");
exit(1);
}

printf("server: waiting for connections...\n");

while(1) { // main accept() loop
sin_size = sizeof their_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
if (new_fd == -1) {
perror("accept");
continue;
}

inet_ntop(their_addr.ss_family,
get_in_addr((struct sockaddr *)&their_addr),
s, sizeof s);
printf("server: got connection from %s\n", s);

if (!fork()) { // this is the child process
close(sockfd); // child doesn't need the listener
if (send(new_fd, "Hello, world!", 13, 0) == -1)
perror("send");
close(new_fd);
exit(0);
}
close(new_fd); // parent doesn't need this
}

return 0;
}
```

**Versions:**

- drrun version 7.91.18323
- gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
- Linux host 5.3.0-26-generic #28~18.04.1-Ubuntu SMP Wed Dec 18 16:40:14 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
- gdb 8.1-0ubuntu3.2

**App built via:**

`gcc -m32 server.c -o server`

**Execution via X86_32:**

```
/home/mg/D.I.C.E/core/src/tracer/dynamorio/dynamorio-linux/build32/bin32/drrun -32 -debug -loglevel 4 -- /home/mg/tmp/server

server: waiting for connections...
server: got connection from 127.0.0.1
<-- parent 4122 forked child 4168 -->

Segmentation fault
```

and `nc localhost 3490` from a second shell to trigger the fork.

**Additional context:**

This problem did not appear on X86_64. For further details see the original discussion: https://groups.google.com/forum/#!topic/dynamorio-users/G_m3wqSxz9w ("Running an app that uses a SIGCHLD handler for zombie process reaping")