scgi_run.c

/*
Copyright (c) 2011, Barry Pederson <bp@barryp.org>
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

    1. Redistributions of source code must retain the above copyright
       notice, this list of conditions and the following disclaimer.

    2. Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in the
       documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

/*
 Execute a CGI script over a SCGI connection

 Compile and strip with:

    cc -o scgi_run scgi_run.c
    strip scgi_run

 For optional debugging, define a DEBUG_FILENAME macro as in:

    cc -o scgi_run -DDEBUG_FILENAME=/tmp/foo.log scgi_run.c

 2011-03-30 Barry Pederson <bp@barryp.org>

*/
#include <ctype.h>
#include <errno.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

extern char **environ;
extern int errno;

#define MAX_HEADER_LENGTH 262144  // just for a sanity check on what the server sends

/**
 Debugging code, requires stdarg.h, stdio.h, and time.h

**/
#ifdef DEBUG_FILENAME
    #define QUOTE(name) #name
    #define STR(macro) QUOTE(macro)

    static void debug_log(const char *msg, ...) {
        char timestamp[32]; // really only need 21 bytes here
        time_t now;
        va_list ap;
        FILE *f;

        now = time(NULL);
        strftime(timestamp, sizeof(timestamp), "%Y-%m-%d %H:%M:%S ", localtime(&now));

        f = fopen(STR(DEBUG_FILENAME), "a");

        fputs(timestamp, f);

        va_start(ap, msg);
        vfprintf(f, msg, ap);
        va_end(ap);

        fputc('\n', f);
        fclose(f);
    }
#else
    #define debug_log
#endif


void return_error(char *status_msg, char *msg, ...) {
    va_list ap;
    char *formatted_msg;

    va_start(ap, msg);
    vasprintf(&formatted_msg, msg, ap);
    va_end(ap);

    fprintf(stdout, "Status: %s\r\nContent-Type: text/plain\r\n\r\n%s\r\n",
        status_msg, formatted_msg);
    exit(1);
}


int read_char() {
    int ch;

    if (read(0, &ch, 1) != 1)
        return_error("500 Internal Error", "SCGI stream truncated");
    return ch;
}


char * read_scgi_environment() {
    char *headers;
    char *name;
    char *value;
    int length;
    char ch;

    ch = read_char();
    if (isdigit(ch)) {
        length = ch - '0';
    } else {
        return_error("500 Internal Error", "SCGI stream didn't start with a digit, started with char 0x%x", ch);
    }

    while (1) {
        ch = read_char();
        if (isdigit(ch)) {
            length = (length * 10) + (ch - '0');
            if (length < 0 || length > MAX_HEADER_LENGTH) {
                return_error("500 Internal Error", "SCGI Header length is not in the range 0..%d", MAX_HEADER_LENGTH);
            }
        } else {
            if (ch == ':') {
                break;
            } else {
               return_error("500 Invalid SCGI header", "Invalid character 0x%x in length", ch);
            }
        }
    }

    debug_log("SCGI header length = %d", length);

    headers = (char *) malloc(length+1);  // +1 is for the comma after the headers
    if (!headers) {
        return_error("500 Internal Error", "Can't allocate memory to read SCGI header");
    }

    if (read(0, headers, length+1) != length+1) {
        return_error("500 Internal Error", "SCGI Header truncated");
    }

    if (headers[length] != ',') {
        return_error("500 Internal Error", "SCGI Header: Incomplete netstring, missing comma");
    }

    if (length) {
        name = headers;
        while (name < (headers + length)) {
            value = name + strlen(name) + 1;
            if (value >= (headers + length))
                return_error("500 Internal Error", "SCGI Header: Corrupt name/value table");
            setenv(name, value, 1);
            debug_log("Set [%s]=[%s]", name, value);
            name = value + strlen(value) + 1;
        }
    }
}

int main(int argc, char **argv) {
    int i;
    char *script_filename;
    char *check_directory = NULL;
    char *new_argv[2];
    char **exec_args = new_argv;

    debug_log("-------- Starting, argc=%d", argc);
    for (i = 0; i < argc; i++) {
        debug_log("argv[%d] == [%s]", i, argv[i]);
    }

    read_scgi_environment();

    /* Make things look like a CGI environment */
    unsetenv("SCGI");
    setenv("GATEWAY_INTERFACE", "CGI/1.1", 1);

    script_filename = getenv("SCRIPT_FILENAME");
    new_argv[0] = script_filename;
    new_argv[1] = NULL;

    if (argc > 1) {
        if (argv[1][strlen(argv[1]) - 1] == '/') {
            check_directory = argv[1];
        } else {
            /* specific script and maybe args in inetd config */
            script_filename = argv[1];
            exec_args = &argv[1];
        }
    }

    if (!script_filename) {
        return_error("500 Internal Error", "CGI environment missing SCRIPT_FILENAME");
    }

    if (strstr(script_filename, "../")) {
        return_error("500 Internal Error", "SCRIPT_FILENAME should not include \"../\"");
    }

    if (check_directory) {
        if (strncmp(check_directory, script_filename, strlen(check_directory))) {
            return_error("500 Internal Error",
                "[%s] doesn't reside under [%s]",
                script_filename, check_directory);
        }
    }

    execve(script_filename, exec_args, environ);

    /* If execve returned, something went wrong */
    if (errno == ENOENT) {
        return_error("404 Not Found", "Can't locate CGI script\n");
    }

    return_error("500 Internal Error",
        "Unable to execute CGI script, please contact the system administrator\n%s\n", strerror(errno));
}