How to determine if code is running in signal-handler context?

Question

I just found out that someone is calling - from a signal handler - a definitely not async-signal-safe function that I wrote.

So, now I\'m curious: how to circumvent this

Answer 1

Apparently, newer Linux/x86 (probably since some 2.6.x kernel) calls signal handlers from the vdso. You could use this fact to inflict the following horrible hack upon the unsuspecting world:

#include 
#include 
#include 
#include 
#include 

#include 

uintmax_t vdso_start = 0;
uintmax_t vdso_end = 0;             /* actually, next byte */

int check_stack_for_vdso(uint32_t *esp, size_t len)
{
    size_t i;

    for (i = 0; i < len; i++, esp++)
            if (*esp >= vdso_start && *esp < vdso_end)
                    return 1;

    return 0;
}

void handler(int signo)
{
    uint32_t *esp;

    __asm__ __volatile__ ("mov %%esp, %0" : "=r"(esp));
    /* XXX only for demonstration, don't call printf from a signal handler */
    printf("handler: check_stack_for_vdso() = %d\n", check_stack_for_vdso(esp, 20));
}

void parse_maps()
{
    FILE *maps;
    char buf[256];
    char path[7];
    uintmax_t start, end, offset, inode;
    char r, w, x, p;
    unsigned major, minor;

    maps = fopen("/proc/self/maps", "rt");
    if (maps == NULL)
            return;

    while (!feof(maps) && !ferror(maps)) {
            if (fgets(buf, 256, maps) != NULL) {
                    if (sscanf(buf, "%jx-%jx %c%c%c%c %jx %u:%u %ju %6s",
                                    &start, &end, &r, &w, &x, &p, &offset,
                                    &major, &minor, &inode, path) == 11) {
                            if (!strcmp(path, "[vdso]")) {
                                    vdso_start = start;
                                    vdso_end = end;
                                    break;
                            }
                    }
            }
    }

    fclose(maps);

    printf("[vdso] at %jx-%jx\n", vdso_start, vdso_end);
}

int main()
{
    struct sigaction sa;
    uint32_t *esp;

    parse_maps();
    memset(&sa, 0, sizeof(struct sigaction));
    sa.sa_handler = handler;
    sa.sa_flags = SA_RESTART;

    if (sigaction(SIGUSR1, &sa, NULL) < 0) {
            perror("sigaction");
            exit(1);
    }

    __asm__ __volatile__ ("mov %%esp, %0" : "=r"(esp));
    printf("before kill: check_stack_for_vdso() = %d\n", check_stack_for_vdso(esp, 20));

    kill(getpid(), SIGUSR1);

    __asm__ __volatile__ ("mov %%esp, %0" : "=r"(esp));
    printf("after kill: check_stack_for_vdso() = %d\n", check_stack_for_vdso(esp, 20));

    return 0;
}

SCNR.