linux_vdso_my_sched_getcpu
Posted by Anonymous on Wed 30th Nov 2022 14:50
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/*
 * linux_vdso_my_sched_getcpu.c - use Linux VDSO
 * shortcut for |sched_getcpu()|, even if userland (g)libc
 * does not have support for VDSO
 *
 * Compile with:
 * $ clang -std=c99 -g -m32 -Wall \
 *      -Wextra linux_vdso_my_sched_getcpu.c -lpthread \
 *      -o linux_vdso_my_sched_getcpu
 *
 * Written by Roland Mainz <roland.mainz@nrubsig.org>
 *
 */

#define _XOPEN_SOURCE   700
#define _GNU_SOURCE     1

#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <elf.h>
#include <time.h>
#include <pthread.h>
#include <sched.h>
#include <sys/auxv.h>

/* Catch cursed idiots playing around with XPG config */
#if (_XOPEN_SOURCE < 600)
#error _XOPEN_SOURCE wrong version
#endif

/* Build test code ? */
#define MY_SCHED_GETCPU_TESTS 1

/* debug */
#if 1
#define D(x) x
#else
#define D(x)
#endif


/* 32bit vs. 64bit */
#if (__PTRDIFF_WIDTH__ == 32)
typedef Elf32_Ehdr      ElfXX_Ehdr;
typedef Elf32_Shdr      ElfXX_Shdr;
typedef Elf32_Sym       ElfXX_Sym;
#elif (__PTRDIFF_WIDTH__ == 64)
typedef Elf64_Ehdr      ElfXX_Ehdr;
typedef Elf64_Shdr      ElfXX_Shdr;
typedef Elf64_Sym       ElfXX_Sym;
#else
#error Unsupported __PTRDIFF_WIDTH__
#endif


/*
 * |get_linux_vdso_sym()| - Look up symbol in AUX AT_SYSINFO_EHDR
 */
static
void *get_linux_vdso_sym(const char *funcname)
{
        const char      *dynstr;
        void            *ret = NULL;
        unsigned char   *vdso_addr;
        ssize_t         i;
        size_t          si;
        const char      *name;
        ElfXX_Ehdr      *elf_header;
        ElfXX_Shdr      *section_header;
        ElfXX_Shdr      *s;
        ElfXX_Shdr      *s2;
        ElfXX_Sym       *sym;

        /*
         * valgrind can disable |AT_SYSINFO_EHDR| - see
         * https://www.mail-archive.com/kde-bugs-dist@kde.org/msg550927.html
         */
        vdso_addr = (unsigned char *)getauxval(AT_SYSINFO_EHDR);
        if (!vdso_addr) {
                D((void)fprintf(stderr,
                        "get_linux_vdso_sym: getauxval(AT_SYSINFO_EHDR) failed.\n"));
                return NULL;
        }

        elf_header = (ElfXX_Ehdr *)vdso_addr;
        section_header = (ElfXX_Shdr *)(vdso_addr + elf_header->e_shoff);

        for (dynstr = NULL, i=0L; i < elf_header->e_shnum; i++) {
                s    = &section_header[i];
                s2   = &section_header[elf_header->e_shstrndx];
                name = (const char*)(vdso_addr + s2->sh_offset + s->sh_name);

                if (strcmp(name, ".dynstr") == 0) {
                        dynstr = (const char *)(vdso_addr + s->sh_offset);
                        break;
                }
        }

        if (!dynstr)
                return NULL;

        for (i=0L; i < elf_header->e_shnum; i++) {
                s    = &section_header[i];
                s2   = &section_header[elf_header->e_shstrndx];
                name = (const char*)(vdso_addr + s2->sh_offset + s->sh_name);

                /* Not .dynsym ? Then look at next section... */
                if (strcmp(name, ".dynsym"))
                        continue;

                for (si=0L; si < (s->sh_size/s->sh_entsize); si++) {
                        sym = &(((ElfXX_Sym*)(vdso_addr + s->sh_offset))[si]);
                        name = dynstr + sym->st_name;

                        D((void)fprintf(stderr, "get_linux_vdso_sym: "
                                "symbol=|%s| vs. |%s|\n",
                                funcname, name));
                        if (!strcmp(name, funcname)) {
                                ret = (void *)(vdso_addr + sym->st_value);
                                break;
                        }
                }

                if (ret)
                        break;
        }

        return ret;
}


/*
 * Initalisation code
 */

/*
 * Note that the |getcpu()| prototype here differs between
 * Linux versions, the third argument (|tcache|) has been
 * removed, and the prototype was moved from
 * <linux/getcpu.h> to <bits/sched.h>.
 */
typedef int (getcpu_func_t)(unsigned *cpu, unsigned *node, void *tcache);

static pthread_once_t my_getcpu_func_is_initialized = PTHREAD_ONCE_INIT;
static getcpu_func_t *my_getcpu_func = NULL;


static
void my_getcpu_func_init(void)
{
        my_getcpu_func = (getcpu_func_t *)get_linux_vdso_sym("__vdso_getcpu");
        if (!my_getcpu_func)
                my_getcpu_func = (getcpu_func_t *)get_linux_vdso_sym("getcpu");

        D((void)fprintf(stderr, "my_getcpu_func_init: "
                "my_getcpu_func = 0x%lx (%s)\n",
                (long)my_getcpu_func,
                (my_getcpu_func?
                        "Using fast codepath":
                        "Using slow codepath")));
        if (!my_getcpu_func) {
                (void)fprintf(stderr, "my_getcpu_func_init: "
                        "WARNING: aux symbol for getcpu not found"
                        ", using slow libc codepath.\n");
        }
}


/*
 * |my_sched_getcpu()| - our "public" interface
 *
 * Notes:
 * - The first call might be slower because the call to
 * |my_getcpu_func_init()|
 */
int my_sched_getcpu(void)
{
        unsigned int    cpu;
        int             s;

        (void)pthread_once(&my_getcpu_func_is_initialized,
                my_getcpu_func_init);

        if (my_getcpu_func) {
                /* our MY_getcpu_TESTS own "fast" codepath */
                s = (*my_getcpu_func)(&cpu, NULL, NULL);
        }
        else
        {
                /*
                 * libc codepath, whatever they use - might be VDSO,
                 * might be syscall
                 *
                 * Note that the prototype here differs between Linux
                 * versions, the third argument (|tcache|) has been
                 * removed, and the prototype was moved from
                 * <linux/getcpu.h> to <bits/sched.h>.
                 * Depending on glibc version this might cause
                 * crashes.
                 */
                s = getcpu(&cpu, NULL/*, NULL*/);
        }

        if (s == -1)
                return -1;

        return (int)cpu;
}


#ifdef MY_SCHED_GETCPU_TESTS
/*
 * test code
 */
static
void test_my_sched_getcpu1(void)
{
        int cpu_num;

        cpu_num = my_sched_getcpu();
        (void)printf("my_sched_getcpu() = %d\n", cpu_num);

        cpu_num = sched_getcpu();
        (void)printf("sched_getcpu()    = %d\n", cpu_num);
}


int main(int ac, char *av[])
{
        (void)ac;
        (void)av;

        test_my_sched_getcpu1();

        return EXIT_SUCCESS;
}

#endif /* MY_SCHED_GETCPU_TESTS */