fix dynamic loader library mapping for nommu systems

on linux/nommu, non-writable private mappings of files may actually
use memory shared with other processes or the fs cache. the old nommu
loader code (used when mmap with MAP_FIXED fails) simply wrote over
top of the original file mapping, possibly clobbering this shared
memory. no such breakage was observed in practice, but it should have
been possible.

the new code starts by mapping anonymous writable memory on archs that
might support nommu, then maps load segments over top of it, falling
back to read if MAP_FIXED fails. we use an anonymous map rather than a
writable file map to avoid reading more data from disk than needed.
since pages cannot be loaded lazily on fault, in case of large
data/bss, mapping the full file may read a lot of data that will
subsequently be thrown away when processing additional LOAD segments.
as a result, we cannot skip the first LOAD segment when operating in
this mode.

these changes affect only non-FDPIC nommu support.

2 files changed, 19 insertions, 4 deletions

diff --git a/src/internal/dynlink.h b/src/internal/dynlink.h
index 86f379e6..9c494e43 100644
--- a/src/internal/dynlink.h
+++ b/src/internal/dynlink.h
@@ -64,6 +64,10 @@ struct fdpic_dummy_loadmap {
 #define DL_FDPIC 0
 #endif
 
+#ifndef DL_NOMMU_SUPPORT
+#define DL_NOMMU_SUPPORT 0
+#endif
+
 #if !DL_FDPIC
 #define IS_RELATIVE(x,s) ( \
 	(R_TYPE(x) == REL_RELATIVE) || \
diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
index a6484dd5..5fbe2bb5 100644
--- a/src/ldso/dynlink.c
+++ b/src/ldso/dynlink.c
@@ -482,8 +482,14 @@ static void reclaim_gaps(struct dso *dso)
 
 static void *mmap_fixed(void *p, size_t n, int prot, int flags, int fd, off_t off)
 {
-	char *q = mmap(p, n, prot, flags, fd, off);
-	if (q != MAP_FAILED || errno != EINVAL) return q;
+	static int no_map_fixed;
+	char *q;
+	if (!no_map_fixed) {
+		q = mmap(p, n, prot, flags|MAP_FIXED, fd, off);
+		if (!DL_NOMMU_SUPPORT || q != MAP_FAILED || errno != EINVAL)
+			return q;
+		no_map_fixed = 1;
+	}
 	/* Fallbacks for MAP_FIXED failure on NOMMU kernels. */
 	if (flags & MAP_ANONYMOUS) {
 		memset(p, 0, n);
@@ -631,7 +637,11 @@ static void *map_library(int fd, struct dso *dso)
 	 * the length of the file. This is okay because we will not
 	 * use the invalid part; we just need to reserve the right
 	 * amount of virtual address space to map over later. */
-	map = mmap((void *)addr_min, map_len, prot, MAP_PRIVATE, fd, off_start);
+	map = DL_NOMMU_SUPPORT
+		? mmap((void *)addr_min, map_len, PROT_READ|PROT_WRITE|PROT_EXEC,
+			MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
+		: mmap((void *)addr_min, map_len, prot,
+			MAP_PRIVATE, fd, off_start);
 	if (map==MAP_FAILED) goto error;
 	dso->map = map;
 	dso->map_len = map_len;
@@ -656,7 +666,8 @@ static void *map_library(int fd, struct dso *dso)
 			dso->phentsize = eh->e_phentsize;
 		}
 		/* Reuse the existing mapping for the lowest-address LOAD */
-		if ((ph->p_vaddr & -PAGE_SIZE) == addr_min) continue;
+		if ((ph->p_vaddr & -PAGE_SIZE) == addr_min && !DL_NOMMU_SUPPORT)
+			continue;
 		this_min = ph->p_vaddr & -PAGE_SIZE;
 		this_max = ph->p_vaddr+ph->p_memsz+PAGE_SIZE-1 & -PAGE_SIZE;
 		off_start = ph->p_offset & -PAGE_SIZE;