reclaim the memory wasted by dynamic linking for use by malloc

diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
index 527b36bd12cdc6c01764e7a6cf79e470300d4d2a..031e1f0f22f51ecabf8cb39638f7565e6e25c947 100644 (file)
--- a/src/ldso/dynlink.c
+++ b/src/ldso/dynlink.c
@@ -139,6 +139,37 @@ static void do_relocs(unsigned char *base, size_t *rel, size_t rel_size, size_t
        }
 }
 
+/* A huge hack: to make up for the wastefulness of shared libraries
+ * needing at least a page of dirty memory even if they have no global
+ * data, we reclaim the gaps at the beginning and end of writable maps
+ * and "donate" them to the heap by setting up minimal malloc
+ * structures and then freeing them. */
+
+static void reclaim(unsigned char *base, size_t start, size_t end)
+{
+       size_t *a, *z;
+       start = start + 6*sizeof(size_t)-1 & -4*sizeof(size_t);
+       end = (end & -4*sizeof(size_t)) - 2*sizeof(size_t);
+       if (start>end || end-start < 4*sizeof(size_t)) return;
+       a = (size_t *)(base + start);
+       z = (size_t *)(base + end);
+       a[-2] = 1;
+       a[-1] = z[0] = end-start + 2*sizeof(size_t) | 1;
+       z[1] = 1;
+       free(a);
+}
+
+static void reclaim_gaps(unsigned char *base, Phdr *ph, size_t phent, size_t phcnt)
+{
+       for (; phcnt--; ph=(void *)((char *)ph+phent)) {
+               if (ph->p_type!=PT_LOAD) continue;
+               if ((ph->p_flags&(PF_R|PF_W))!=(PF_R|PF_W)) continue;
+               reclaim(base, ph->p_vaddr & -PAGE_SIZE, ph->p_vaddr);
+               reclaim(base, ph->p_vaddr+ph->p_memsz,
+                       ph->p_vaddr+ph->p_memsz+PAGE_SIZE-1 & -PAGE_SIZE);
+       }
+}
+
 static void *map_library(int fd, size_t *lenp, unsigned char **basep, size_t *dynp)
 {
        Ehdr buf[(896+sizeof(Ehdr))/sizeof(Ehdr)];
@@ -217,6 +248,8 @@ static void *map_library(int fd, size_t *lenp, unsigned char **basep, size_t *dy
                        }
                }
        }
+       if (!runtime) reclaim_gaps(base, (void *)((char *)buf + eh->e_phoff),
+               eh->e_phentsize, eh->e_phnum);
        *lenp = map_len;
        *basep = base;
        *dynp = dyn;
@@ -398,6 +431,7 @@ void *__dynlink(int argc, char **argv, size_t *got)
        size_t lib_dyn[DYN_CNT] = {0};
        size_t i;
        Phdr *phdr;
+       Ehdr *ehdr;
        struct dso lib, app;
 
        /* Find aux vector just past environ[] */
@@ -456,6 +490,11 @@ void *__dynlink(int argc, char **argv, size_t *got)
 
        /* At this point the standard library is fully functional */
 
+       reclaim_gaps(app.base, (void *)aux[AT_PHDR], aux[AT_PHENT], aux[AT_PHNUM]);
+       ehdr = (void *)lib.base;
+       reclaim_gaps(lib.base, (void *)(lib.base+ehdr->e_phoff),
+               ehdr->e_phentsize, ehdr->e_phnum);
+
        head = tail = &app;
        libc = &lib;
        app.next = 0;