modutils: add FEATURE_INSMOD_TRY_MMAP option

function                                             old     new   delta
try_to_mmap_module                                     -     121    +121
bb_init_module_24                                   4514    4578     +64
bb_init_module                                       119     173     +54
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/0 up/down: 239/0)             Total: 239 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

4 files changed, 93 insertions, 17 deletions

diff --git a/modutils/Config.in b/modutils/Config.in
index d2a2e04d1..83c12b67f 100644
--- a/modutils/Config.in
+++ b/modutils/Config.in
@@ -121,6 +121,22 @@ config FEATURE_2_4_MODULES
           This increases size considerably. Say N unless you plan
           to run ancient kernels.
 
+config FEATURE_INSMOD_TRY_MMAP
+        bool "Try to load module from a mmap'ed area"
+        default n
+        depends on INSMOD || MODPROBE_SMALL
+        help
+          This option causes module loading code to try to mmap
+          module first. If it does not work (for example,
+          it does not work for compressed modules), module will be read
+          (and unpacked if needed) into a memory block allocated by malloc.
+
+          The only case when mmap works but malloc does not is when
+          you are trying to load a big module on a very memory-constrained
+          machine. Malloc will momentarily need 2x as much memory as mmap.
+
+          Choosing N saves about 250 bytes of code (on 32-bit x86).
+
 config FEATURE_INSMOD_VERSION_CHECKING
         bool "Enable module version checking"
         default n
diff --git a/modutils/modutils-24.c b/modutils/modutils-24.c
index e5ff54d29..a878e740c 100644
--- a/modutils/modutils-24.c
+++ b/modutils/modutils-24.c
@@ -3783,12 +3783,20 @@ int FAST_FUNC bb_init_module_24(const char *m_filename, const char *options)
         int m_has_modinfo;
 #endif
         char *image;
-        size_t image_size = 64 * 1024 * 1024;
-
-        /* Load module into memory and unzip if compressed */
-        image = xmalloc_open_zipped_read_close(m_filename, &image_size);
-        if (!image)
-                return EXIT_FAILURE;
+        size_t image_size;
+        bool mmaped;
+
+        image_size = INT_MAX - 4095;
+        mmaped = 0;
+        image = try_to_mmap_module(m_filename, &image_size);
+        if (image) {
+                mmaped = 1;
+        } else {
+                /* Load module into memory and unzip if compressed */
+                image = xmalloc_open_zipped_read_close(m_filename, &image_size);
+                if (!image)
+                        return EXIT_FAILURE;
+        }
 
         m_name = xstrdup(bb_basename(m_filename));
         /* "module.o[.gz]" -> "module" */
@@ -3901,7 +3909,10 @@ int FAST_FUNC bb_init_module_24(const char *m_filename, const char *options)
         exit_status = EXIT_SUCCESS;
 
  out:
-        free(image);
+        if (mmaped)
+                munmap(image, image_size);
+        else
+                free(image);
         free(m_name);
 
         return exit_status;
diff --git a/modutils/modutils.c b/modutils/modutils.c
index 969926db9..850a8683b 100644
--- a/modutils/modutils.c
+++ b/modutils/modutils.c
@@ -62,7 +62,7 @@ char* FAST_FUNC filename2modname(const char *filename, char *modname)
         return modname;
 }
 
-char * FAST_FUNC parse_cmdline_module_options(char **argv)
+char* FAST_FUNC parse_cmdline_module_options(char **argv)
 {
         char *options;
         int optlen;
@@ -77,6 +77,40 @@ char * FAST_FUNC parse_cmdline_module_options(char **argv)
         return options;
 }
 
+#if ENABLE_FEATURE_INSMOD_TRY_MMAP
+void* FAST_FUNC try_to_mmap_module(const char *filename, size_t *image_size_p)
+{
+        /* We have user reports of failure to load 3MB module
+         * on a 16MB RAM machine. Apparently even a transient
+         * memory spike to 6MB during module load
+         * is too big for that system. */
+        void *image;
+        struct stat st;
+        int fd;
+
+        fd = xopen(filename, O_RDONLY);
+        fstat(fd, &st);
+        image = NULL;
+        /* st.st_size is off_t, we can't just pass it to mmap */
+        if (st.st_size <= *image_size_p) {
+                size_t image_size = st.st_size;
+                image = mmap(NULL, image_size, PROT_READ, MAP_PRIVATE, fd, 0);
+                if (image == MAP_FAILED) {
+                        image = NULL;
+                } else if (*(uint32_t*)image != SWAP_BE32(0x7f454C46)) {
+                        /* No ELF signature. Compressed module? */
+                        munmap(image, image_size);
+                        image = NULL;
+                } else {
+                        /* Success. Report the size */
+                        *image_size_p = image_size;
+                }
+        }
+        close(fd);
+        return image;
+}
+#endif
+
 /* Return:
  * 0 on success,
  * -errno on open/read error,
@@ -84,9 +118,10 @@ char * FAST_FUNC parse_cmdline_module_options(char **argv)
  */
 int FAST_FUNC bb_init_module(const char *filename, const char *options)
 {
-        size_t len;
+        size_t image_size;
         char *image;
         int rc;
+        bool mmaped;
 
         if (!options)
                 options = "";
@@ -97,17 +132,25 @@ int FAST_FUNC bb_init_module(const char *filename, const char *options)
                 return bb_init_module_24(filename, options);
 #endif
 
-        /* Use the 2.6 way */
-        len = INT_MAX - 4095;
-        errno = ENOMEM; /* may be changed by e.g. open errors below */
-        image = xmalloc_open_zipped_read_close(filename, &len);
-        if (!image)
-                return -errno;
+        image_size = INT_MAX - 4095;
+        mmaped = 0;
+        image = try_to_mmap_module(filename, &image_size);
+        if (image) {
+                mmaped = 1;
+        } else {
+                errno = ENOMEM; /* may be changed by e.g. open errors below */
+                image = xmalloc_open_zipped_read_close(filename, &image_size);
+                if (!image)
+                        return -errno;
+        }
 
         errno = 0;
-        init_module(image, len, options);
+        init_module(image, image_size, options);
         rc = errno;
-        free(image);
+        if (mmaped)
+                munmap(image, image_size);
+        else
+                free(image);
         return rc;
 }
 
diff --git a/modutils/modutils.h b/modutils/modutils.h
index 1cf4bba95..131a5087b 100644
--- a/modutils/modutils.h
+++ b/modutils/modutils.h
@@ -51,6 +51,12 @@ enum {
 #endif
 };
 
+#if ENABLE_FEATURE_INSMOD_TRY_MMAP
+void* FAST_FUNC try_to_mmap_module(const char *filename, size_t *image_size_p);
+#else
+# define try_to_mmap_module(filename, image_size) NULL
+#endif
+
 /* Return:
  * 0 on success,
  * -errno on open/read error,