Use mmap to read an initial ELF header of library

Also, fixes SIGSEV during calculation of si->phdr in load_library(),
which is caused by accessing PHDR area, which is not guaranteed to be
loaded.

Note that this usually does not happen because program header area is
covered by immediately following PT_LOAD entry.  But it does not hold
always.

Bug: 6561258
Change-Id: Ie376253c773b5420279ca89044e81b1aad5a5736

1 files changed, 17 insertions, 18 deletions

diff --git a/linker/linker.c b/linker/linker.c
index eb9cc3e..739d3e4 100644
--- a/linker/linker.c
+++ b/linker/linker.c
@@ -640,10 +640,6 @@ static int open_library(const char *name)
     return -1;
 }
 
-/* temporary space for holding the first page of the shared lib
- * which contains the elf header (with the pht). */
-static unsigned char __header[PAGE_SIZE];
-
 typedef struct {
     long mmap_addr;
     char tag[4]; /* 'P', 'R', 'E', ' ' */
@@ -1086,29 +1082,34 @@ load_library(const char *name)
     unsigned ext_sz;
     unsigned req_base;
     const char *bname;
+    struct stat sb;
     soinfo *si = NULL;
-    Elf32_Ehdr *hdr;
+    Elf32_Ehdr *hdr = MAP_FAILED;
 
-    if(fd == -1) {
+    if (fd == -1) {
         DL_ERR("Library '%s' not found", name);
         return NULL;
     }
 
-    /* We have to read the ELF header to figure out what to do with this image
+    /* We have to read the ELF header to figure out what to do with this image.
+     * Map entire file for this.  There won't be much difference in physical
+     * memory usage or performance.
      */
-    if (lseek(fd, 0, SEEK_SET) < 0) {
-        DL_ERR("lseek() failed!");
+    if (fstat(fd, &sb) < 0) {
+        DL_ERR("%5d fstat() failed! (%s)", pid, strerror(errno));
         goto fail;
     }
 
-    if ((cnt = read(fd, &__header[0], PAGE_SIZE)) < 0) {
-        DL_ERR("read() failed!");
+    hdr = mmap(NULL, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+    if (hdr == MAP_FAILED) {
+        DL_ERR("%5d failed to mmap() header of '%s' (%s)",
+               pid, name, strerror(errno));
         goto fail;
     }
 
     /* Parse the ELF header and get the size of the memory footprint for
      * the library */
-    req_base = get_lib_extents(fd, name, &__header[0], &ext_sz);
+    req_base = get_lib_extents(fd, name, hdr, &ext_sz);
     if (req_base == (unsigned)-1)
         goto fail;
     TRACE("[ %5d - '%s' (%s) wants base=0x%08x sz=0x%08x ]\n", pid, name,
@@ -1138,22 +1139,20 @@ load_library(const char *name)
           pid, name, (void *)si->base, (unsigned) ext_sz);
 
     /* Now actually load the library's segments into right places in memory */
-    if (load_segments(fd, &__header[0], si) < 0) {
+    if (load_segments(fd, hdr, si) < 0) {
         goto fail;
     }
 
-    /* this might not be right. Technically, we don't even need this info
-     * once we go through 'load_segments'. */
-    hdr = (Elf32_Ehdr *)si->base;
     si->phdr = (Elf32_Phdr *)((unsigned char *)si->base + hdr->e_phoff);
     si->phnum = hdr->e_phnum;
-    /**/
 
+    munmap(hdr, sb.st_size);
     close(fd);
     return si;
 
 fail:
     if (si) free_info(si);
+    if (hdr != MAP_FAILED) munmap(hdr, sb.st_size);
     close(fd);
     return NULL;
 }
@@ -1678,7 +1677,7 @@ static int link_image(soinfo *si, unsigned wr_offset)
 
     if (si->flags & (FLAG_EXE | FLAG_LINKER)) {
         /* Locate the needed program segments (DYNAMIC/ARM_EXIDX) for
-         * linkage info if this is the executable or the linker itself. 
+         * linkage info if this is the executable or the linker itself.
          * If this was a dynamic lib, that would have been done at load time.
          *
          * TODO: It's unfortunate that small pieces of this are