exec.c

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/*-------------------------------------------------------------------------
 *
 * exec.c
 *      Functions for finding and validating executable files
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/common/exec.c
 *
 *-------------------------------------------------------------------------
 */

#ifndef FRONTEND
#include "postgres.h"
#else
#include "postgres_fe.h"
#endif

#include <signal.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <unistd.h>

/*
 * Hacky solution to allow expressing both frontend and backend error reports
 * in one macro call.  First argument of log_error is an errcode() call of
 * some sort (ignored if FRONTEND); the rest are errmsg_internal() arguments,
 * i.e. message string and any parameters for it.
 *
 * Caller must provide the gettext wrapper around the message string, if
 * appropriate, so that it gets translated in the FRONTEND case; this
 * motivates using errmsg_internal() not errmsg().  We handle appending a
 * newline, if needed, inside the macro, so that there's only one translatable
 * string per call not two.
 */
#ifndef FRONTEND
#define log_error(errcodefn, ...) \
    ereport(LOG, (errcodefn, errmsg_internal(__VA_ARGS__)))
#else
#define log_error(errcodefn, ...) \
    (fprintf(stderr, __VA_ARGS__), fputc('\n', stderr))
#endif

#ifdef _MSC_VER
#define getcwd(cwd,len)  GetCurrentDirectory(len, cwd)
#endif

static int  resolve_symlinks(char *path);

#ifdef WIN32
static BOOL GetTokenUser(HANDLE hToken, PTOKEN_USER *ppTokenUser);
#endif

/*
 * validate_exec -- validate "path" as an executable file
 *
 * returns 0 if the file is found and no error is encountered.
 *        -1 if the regular file "path" does not exist or cannot be executed.
 *        -2 if the file is otherwise valid but cannot be read.
 */
int
validate_exec(const char *path)
{
    struct stat buf;
    int         is_r;
    int         is_x;

#ifdef WIN32
    char        path_exe[MAXPGPATH + sizeof(".exe") - 1];

    /* Win32 requires a .exe suffix for stat() */
    if (strlen(path) >= strlen(".exe") &&
        pg_strcasecmp(path + strlen(path) - strlen(".exe"), ".exe") != 0)
    {
        strlcpy(path_exe, path, sizeof(path_exe) - 4);
        strcat(path_exe, ".exe");
        path = path_exe;
    }
#endif

    /*
     * Ensure that the file exists and is a regular file.
     *
     * XXX if you have a broken system where stat() looks at the symlink
     * instead of the underlying file, you lose.
     */
    if (stat(path, &buf) < 0)
        return -1;

    if (!S_ISREG(buf.st_mode))
        return -1;

    /*
     * Ensure that the file is both executable and readable (required for
     * dynamic loading).
     */
#ifndef WIN32
    is_r = (access(path, R_OK) == 0);
    is_x = (access(path, X_OK) == 0);
#else
    is_r = buf.st_mode & S_IRUSR;
    is_x = buf.st_mode & S_IXUSR;
#endif
    return is_x ? (is_r ? 0 : -2) : -1;
}


/*
 * find_my_exec -- find an absolute path to a valid executable
 *
 *  argv0 is the name passed on the command line
 *  retpath is the output area (must be of size MAXPGPATH)
 *  Returns 0 if OK, -1 if error.
 *
 * The reason we have to work so hard to find an absolute path is that
 * on some platforms we can't do dynamic loading unless we know the
 * executable's location.  Also, we need a full path not a relative
 * path because we will later change working directory.  Finally, we want
 * a true path not a symlink location, so that we can locate other files
 * that are part of our installation relative to the executable.
 */
int
find_my_exec(const char *argv0, char *retpath)
{
    char        cwd[MAXPGPATH],
                test_path[MAXPGPATH];
    char       *path;

    if (!getcwd(cwd, MAXPGPATH))
    {
        log_error(errcode_for_file_access(),
                  _("could not identify current directory: %m"));
        return -1;
    }

    /*
     * If argv0 contains a separator, then PATH wasn't used.
     */
    if (first_dir_separator(argv0) != NULL)
    {
        if (is_absolute_path(argv0))
            strlcpy(retpath, argv0, MAXPGPATH);
        else
            join_path_components(retpath, cwd, argv0);
        canonicalize_path(retpath);

        if (validate_exec(retpath) == 0)
            return resolve_symlinks(retpath);

        log_error(errcode(ERRCODE_WRONG_OBJECT_TYPE),
                  _("invalid binary \"%s\""), retpath);
        return -1;
    }

#ifdef WIN32
    /* Win32 checks the current directory first for names without slashes */
    join_path_components(retpath, cwd, argv0);
    if (validate_exec(retpath) == 0)
        return resolve_symlinks(retpath);
#endif

    /*
     * Since no explicit path was supplied, the user must have been relying on
     * PATH.  We'll search the same PATH.
     */
    if ((path = getenv("PATH")) && *path)
    {
        char       *startp = NULL,
                   *endp = NULL;

        do
        {
            if (!startp)
                startp = path;
            else
                startp = endp + 1;

            endp = first_path_var_separator(startp);
            if (!endp)
                endp = startp + strlen(startp); /* point to end */

            strlcpy(test_path, startp, Min(endp - startp + 1, MAXPGPATH));

            if (is_absolute_path(test_path))
                join_path_components(retpath, test_path, argv0);
            else
            {
                join_path_components(retpath, cwd, test_path);
                join_path_components(retpath, retpath, argv0);
            }
            canonicalize_path(retpath);

            switch (validate_exec(retpath))
            {
                case 0:         /* found ok */
                    return resolve_symlinks(retpath);
                case -1:        /* wasn't even a candidate, keep looking */
                    break;
                case -2:        /* found but disqualified */
                    log_error(errcode(ERRCODE_WRONG_OBJECT_TYPE),
                              _("could not read binary \"%s\""),
                              retpath);
                    break;
            }
        } while (*endp);
    }

    log_error(errcode(ERRCODE_UNDEFINED_FILE),
              _("could not find a \"%s\" to execute"), argv0);
    return -1;
}


/*
 * resolve_symlinks - resolve symlinks to the underlying file
 *
 * Replace "path" by the absolute path to the referenced file.
 *
 * Returns 0 if OK, -1 if error.
 *
 * Note: we are not particularly tense about producing nice error messages
 * because we are not really expecting error here; we just determined that
 * the symlink does point to a valid executable.
 */
static int
resolve_symlinks(char *path)
{
#ifdef HAVE_READLINK
    struct stat buf;
    char        orig_wd[MAXPGPATH],
                link_buf[MAXPGPATH];
    char       *fname;

    /*
     * To resolve a symlink properly, we have to chdir into its directory and
     * then chdir to where the symlink points; otherwise we may fail to
     * resolve relative links correctly (consider cases involving mount
     * points, for example).  After following the final symlink, we use
     * getcwd() to figure out where the heck we're at.
     *
     * One might think we could skip all this if path doesn't point to a
     * symlink to start with, but that's wrong.  We also want to get rid of
     * any directory symlinks that are present in the given path. We expect
     * getcwd() to give us an accurate, symlink-free path.
     */
    if (!getcwd(orig_wd, MAXPGPATH))
    {
        log_error(errcode_for_file_access(),
                  _("could not identify current directory: %m"));
        return -1;
    }

    for (;;)
    {
        char       *lsep;
        int         rllen;

        lsep = last_dir_separator(path);
        if (lsep)
        {
            *lsep = '\0';
            if (chdir(path) == -1)
            {
                log_error(errcode_for_file_access(),
                          _("could not change directory to \"%s\": %m"), path);
                return -1;
            }
            fname = lsep + 1;
        }
        else
            fname = path;

        if (lstat(fname, &buf) < 0 ||
            !S_ISLNK(buf.st_mode))
            break;

        errno = 0;
        rllen = readlink(fname, link_buf, sizeof(link_buf));
        if (rllen < 0 || rllen >= sizeof(link_buf))
        {
            log_error(errcode_for_file_access(),
                      _("could not read symbolic link \"%s\": %m"), fname);
            return -1;
        }
        link_buf[rllen] = '\0';
        strcpy(path, link_buf);
    }

    /* must copy final component out of 'path' temporarily */
    strlcpy(link_buf, fname, sizeof(link_buf));

    if (!getcwd(path, MAXPGPATH))
    {
        log_error(errcode_for_file_access(),
                  _("could not identify current directory: %m"));
        return -1;
    }
    join_path_components(path, path, link_buf);
    canonicalize_path(path);

    if (chdir(orig_wd) == -1)
    {
        log_error(errcode_for_file_access(),
                  _("could not change directory to \"%s\": %m"), orig_wd);
        return -1;
    }
#endif                          /* HAVE_READLINK */

    return 0;
}


/*
 * Find another program in our binary's directory,
 * then make sure it is the proper version.
 */
int
find_other_exec(const char *argv0, const char *target,
                const char *versionstr, char *retpath)
{
    char        cmd[MAXPGPATH];
    char        line[MAXPGPATH];

    if (find_my_exec(argv0, retpath) < 0)
        return -1;

    /* Trim off program name and keep just directory */
    *last_dir_separator(retpath) = '\0';
    canonicalize_path(retpath);

    /* Now append the other program's name */
    snprintf(retpath + strlen(retpath), MAXPGPATH - strlen(retpath),
             "/%s%s", target, EXE);

    if (validate_exec(retpath) != 0)
        return -1;

    snprintf(cmd, sizeof(cmd), "\"%s\" -V", retpath);

    if (!pipe_read_line(cmd, line, sizeof(line)))
        return -1;

    if (strcmp(line, versionstr) != 0)
        return -2;

    return 0;
}


/*
 * Execute a command in a pipe and read the first line from it.
 */
char *
pipe_read_line(char *cmd, char *line, int maxsize)
{
    FILE       *pgver;

    /* flush output buffers in case popen does not... */
    fflush(stdout);
    fflush(stderr);

    errno = 0;
    if ((pgver = popen(cmd, "r")) == NULL)
    {
        perror("popen failure");
        return NULL;
    }

    errno = 0;
    if (fgets(line, maxsize, pgver) == NULL)
    {
        if (feof(pgver))
            fprintf(stderr, "no data was returned by command \"%s\"\n", cmd);
        else
            perror("fgets failure");
        pclose(pgver);          /* no error checking */
        return NULL;
    }

    if (pclose_check(pgver))
        return NULL;

    return line;
}


/*
 * pclose() plus useful error reporting
 */
int
pclose_check(FILE *stream)
{
    int         exitstatus;
    char       *reason;

    exitstatus = pclose(stream);

    if (exitstatus == 0)
        return 0;               /* all is well */

    if (exitstatus == -1)
    {
        /* pclose() itself failed, and hopefully set errno */
        log_error(errcode(ERRCODE_SYSTEM_ERROR),
                  _("pclose failed: %m"));
    }
    else
    {
        reason = wait_result_to_str(exitstatus);
        log_error(errcode(ERRCODE_SYSTEM_ERROR),
                  "%s", reason);
        pfree(reason);
    }
    return exitstatus;
}

/*
 *  set_pglocale_pgservice
 *
 *  Set application-specific locale and service directory
 *
 *  This function takes the value of argv[0] rather than a full path.
 *
 * (You may be wondering why this is in exec.c.  It requires this module's
 * services and doesn't introduce any new dependencies, so this seems as
 * good as anyplace.)
 */
void
set_pglocale_pgservice(const char *argv0, const char *app)
{
    char        path[MAXPGPATH];
    char        my_exec_path[MAXPGPATH];

    /* don't set LC_ALL in the backend */
    if (strcmp(app, PG_TEXTDOMAIN("postgres")) != 0)
    {
        setlocale(LC_ALL, "");

        /*
         * One could make a case for reproducing here PostmasterMain()'s test
         * for whether the process is multithreaded.  Unlike the postmaster,
         * no frontend program calls sigprocmask() or otherwise provides for
         * mutual exclusion between signal handlers.  While frontends using
         * fork(), if multithreaded, are formally exposed to undefined
         * behavior, we have not witnessed a concrete bug.  Therefore,
         * complaining about multithreading here may be mere pedantry.
         */
    }

    if (find_my_exec(argv0, my_exec_path) < 0)
        return;

#ifdef ENABLE_NLS
    get_locale_path(my_exec_path, path);
    bindtextdomain(app, path);
    textdomain(app);
    /* set for libpq to use, but don't override existing setting */
    setenv("PGLOCALEDIR", path, 0);
#endif

    if (getenv("PGSYSCONFDIR") == NULL)
    {
        get_etc_path(my_exec_path, path);
        /* set for libpq to use */
        setenv("PGSYSCONFDIR", path, 0);
    }
}

#ifdef WIN32

/*
 * AddUserToTokenDacl(HANDLE hToken)
 *
 * This function adds the current user account to the restricted
 * token used when we create a restricted process.
 *
 * This is required because of some security changes in Windows
 * that appeared in patches to XP/2K3 and in Vista/2008.
 *
 * On these machines, the Administrator account is not included in
 * the default DACL - you just get Administrators + System. For
 * regular users you get User + System. Because we strip Administrators
 * when we create the restricted token, we are left with only System
 * in the DACL which leads to access denied errors for later CreatePipe()
 * and CreateProcess() calls when running as Administrator.
 *
 * This function fixes this problem by modifying the DACL of the
 * token the process will use, and explicitly re-adding the current
 * user account.  This is still secure because the Administrator account
 * inherits its privileges from the Administrators group - it doesn't
 * have any of its own.
 */
BOOL
AddUserToTokenDacl(HANDLE hToken)
{
    int         i;
    ACL_SIZE_INFORMATION asi;
    ACCESS_ALLOWED_ACE *pace;
    DWORD       dwNewAclSize;
    DWORD       dwSize = 0;
    DWORD       dwTokenInfoLength = 0;
    PACL        pacl = NULL;
    PTOKEN_USER pTokenUser = NULL;
    TOKEN_DEFAULT_DACL tddNew;
    TOKEN_DEFAULT_DACL *ptdd = NULL;
    TOKEN_INFORMATION_CLASS tic = TokenDefaultDacl;
    BOOL        ret = FALSE;

    /* Figure out the buffer size for the DACL info */
    if (!GetTokenInformation(hToken, tic, (LPVOID) NULL, dwTokenInfoLength, &dwSize))
    {
        if (GetLastError() == ERROR_INSUFFICIENT_BUFFER)
        {
            ptdd = (TOKEN_DEFAULT_DACL *) LocalAlloc(LPTR, dwSize);
            if (ptdd == NULL)
            {
                log_error(errcode(ERRCODE_OUT_OF_MEMORY),
                          _("out of memory"));
                goto cleanup;
            }

            if (!GetTokenInformation(hToken, tic, (LPVOID) ptdd, dwSize, &dwSize))
            {
                log_error(errcode(ERRCODE_SYSTEM_ERROR),
                          "could not get token information: error code %lu",
                          GetLastError());
                goto cleanup;
            }
        }
        else
        {
            log_error(errcode(ERRCODE_SYSTEM_ERROR),
                      "could not get token information buffer size: error code %lu",
                      GetLastError());
            goto cleanup;
        }
    }

    /* Get the ACL info */
    if (!GetAclInformation(ptdd->DefaultDacl, (LPVOID) &asi,
                           (DWORD) sizeof(ACL_SIZE_INFORMATION),
                           AclSizeInformation))
    {
        log_error(errcode(ERRCODE_SYSTEM_ERROR),
                  "could not get ACL information: error code %lu",
                  GetLastError());
        goto cleanup;
    }

    /* Get the current user SID */
    if (!GetTokenUser(hToken, &pTokenUser))
        goto cleanup;           /* callee printed a message */

    /* Figure out the size of the new ACL */
    dwNewAclSize = asi.AclBytesInUse + sizeof(ACCESS_ALLOWED_ACE) +
        GetLengthSid(pTokenUser->User.Sid) - sizeof(DWORD);

    /* Allocate the ACL buffer & initialize it */
    pacl = (PACL) LocalAlloc(LPTR, dwNewAclSize);
    if (pacl == NULL)
    {
        log_error(errcode(ERRCODE_OUT_OF_MEMORY),
                  _("out of memory"));
        goto cleanup;
    }

    if (!InitializeAcl(pacl, dwNewAclSize, ACL_REVISION))
    {
        log_error(errcode(ERRCODE_SYSTEM_ERROR),
                  "could not initialize ACL: error code %lu", GetLastError());
        goto cleanup;
    }

    /* Loop through the existing ACEs, and build the new ACL */
    for (i = 0; i < (int) asi.AceCount; i++)
    {
        if (!GetAce(ptdd->DefaultDacl, i, (LPVOID *) &pace))
        {
            log_error(errcode(ERRCODE_SYSTEM_ERROR),
                      "could not get ACE: error code %lu", GetLastError());
            goto cleanup;
        }

        if (!AddAce(pacl, ACL_REVISION, MAXDWORD, pace, ((PACE_HEADER) pace)->AceSize))
        {
            log_error(errcode(ERRCODE_SYSTEM_ERROR),
                      "could not add ACE: error code %lu", GetLastError());
            goto cleanup;
        }
    }

    /* Add the new ACE for the current user */
    if (!AddAccessAllowedAceEx(pacl, ACL_REVISION, OBJECT_INHERIT_ACE, GENERIC_ALL, pTokenUser->User.Sid))
    {
        log_error(errcode(ERRCODE_SYSTEM_ERROR),
                  "could not add access allowed ACE: error code %lu",
                  GetLastError());
        goto cleanup;
    }

    /* Set the new DACL in the token */
    tddNew.DefaultDacl = pacl;

    if (!SetTokenInformation(hToken, tic, (LPVOID) &tddNew, dwNewAclSize))
    {
        log_error(errcode(ERRCODE_SYSTEM_ERROR),
                  "could not set token information: error code %lu",
                  GetLastError());
        goto cleanup;
    }

    ret = TRUE;

cleanup:
    if (pTokenUser)
        LocalFree((HLOCAL) pTokenUser);

    if (pacl)
        LocalFree((HLOCAL) pacl);

    if (ptdd)
        LocalFree((HLOCAL) ptdd);

    return ret;
}

/*
 * GetTokenUser(HANDLE hToken, PTOKEN_USER *ppTokenUser)
 *
 * Get the users token information from a process token.
 *
 * The caller of this function is responsible for calling LocalFree() on the
 * returned TOKEN_USER memory.
 */
static BOOL
GetTokenUser(HANDLE hToken, PTOKEN_USER *ppTokenUser)
{
    DWORD       dwLength;

    *ppTokenUser = NULL;

    if (!GetTokenInformation(hToken,
                             TokenUser,
                             NULL,
                             0,
                             &dwLength))
    {
        if (GetLastError() == ERROR_INSUFFICIENT_BUFFER)
        {
            *ppTokenUser = (PTOKEN_USER) LocalAlloc(LPTR, dwLength);

            if (*ppTokenUser == NULL)
            {
                log_error(errcode(ERRCODE_OUT_OF_MEMORY),
                          _("out of memory"));
                return FALSE;
            }
        }
        else
        {
            log_error(errcode(ERRCODE_SYSTEM_ERROR),
                      "could not get token information buffer size: error code %lu",
                      GetLastError());
            return FALSE;
        }
    }

    if (!GetTokenInformation(hToken,
                             TokenUser,
                             *ppTokenUser,
                             dwLength,
                             &dwLength))
    {
        LocalFree(*ppTokenUser);
        *ppTokenUser = NULL;

        log_error(errcode(ERRCODE_SYSTEM_ERROR),
                  "could not get token information: error code %lu",
                  GetLastError());
        return FALSE;
    }

    /* Memory in *ppTokenUser is LocalFree():d by the caller */
    return TRUE;
}

#endif