[PATCH] linux-user: Fix loading of BSS segments

[Giuseppe Musacchio]

Some ELF binaries encode the .bss section as an extension of the data
ones by setting the segment p_memsz > p_filesz. Some other binaries take
a different route and encode it as a stand-alone PT_LOAD segment with
p_filesz = 0 and p_memsz > 0.

Both the encodings are actually correct per ELF specification but the
ELF loader had some troubles in handling the former: with the old logic
it was very likely to get Qemu to crash in zero_bss when trying to
access unmapped memory.

zero_bss isn't meant to allocate whole zero-filled segments but to
"complete" a previously mapped segment with the needed zero bits.

The fix is pretty simple, if the segment is completely zero-filled we
simply allocate one or more pages (according to p_memsz) and avoid
calling zero_bss altogether.

Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
---
 linux-user/elfload.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 0b02a92602..a16c240e0f 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -2776,14 +2776,16 @@ static void load_elf_image(const char *image_name, int 
image_fd,
             vaddr = load_bias + eppnt->p_vaddr;
             vaddr_po = TARGET_ELF_PAGEOFFSET(vaddr);
             vaddr_ps = TARGET_ELF_PAGESTART(vaddr);
-            vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz + vaddr_po);
+
+            vaddr_ef = vaddr + eppnt->p_filesz;
+            vaddr_em = vaddr + eppnt->p_memsz;
 
             /*
-             * Some segments may be completely empty without any backing file
-             * segment, in that case just let zero_bss allocate an empty buffer
-             * for it.
+             * Some segments may be completely empty, with a non-zero p_memsz
+             * but no backing file segment.
              */
             if (eppnt->p_filesz != 0) {
+                vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz + vaddr_po);
                 error = target_mmap(vaddr_ps, vaddr_len, elf_prot,
                                     MAP_PRIVATE | MAP_FIXED,
                                     image_fd, eppnt->p_offset - vaddr_po);
@@ -2791,14 +2793,22 @@ static void load_elf_image(const char *image_name, int 
image_fd,
                 if (error == -1) {
                     goto exit_mmap;
                 }
-            }
 
-            vaddr_ef = vaddr + eppnt->p_filesz;
-            vaddr_em = vaddr + eppnt->p_memsz;
+                /*
+                 * If the load segment requests extra zeros (e.g. bss), map it.
+                 */
+                if (eppnt->p_filesz < eppnt->p_memsz) {
+                    zero_bss(vaddr_ef, vaddr_em, elf_prot);
+                }
+            } else if (eppnt->p_memsz != 0) {
+                vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_memsz + vaddr_po);
+                error = target_mmap(vaddr_ps, vaddr_len, elf_prot,
+                                    MAP_PRIVATE | MAP_FIXED | MAP_ANONYMOUS,
+                                    -1, 0);
 
-            /* If the load segment requests extra zeros (e.g. bss), map it.  */
-            if (vaddr_ef < vaddr_em) {
-                zero_bss(vaddr_ef, vaddr_em, elf_prot);
+                if (error == -1) {
+                    goto exit_mmap;
+                }
             }
 
             /* Find the full program boundaries.  */
-- 
2.29.2