Handle empty file size loadable segments

In cases such as those if the application has no .data but has .bss,
the .bss may end up being alone in an empty filesz loadable segment.
Handle such situation by only `mmap`ing file contents if the segment
has actual file contents (filesz != 0).

Signed-off-by: Sergiu Moga <[email protected]>
Approved-by: Razvan Deaconescu <[email protected]>
Reviewed-by: Razvan Deaconescu <[email protected]>
GitHub-Closes: #85

Author: mogasergiu

elf_load.c

@@ -519,19 +519,27 @@ static int do_elf_load_fdphdr_not0(struct elf_prog *elf_prog,
 	/* mmap with all flags. If protections are enabled, these will
 	 * be manually re-adjusted later.
 	 */
-	vastart = (uintptr_t)mmap(addr, phdr->p_filesz + delta_p_offset,
-				  get_phdr_mmap_prot(phdr),
-				  MAP_FIXED | MAP_PRIVATE,
-				  fd, phdr->p_offset - delta_p_offset);
-	if (unlikely(vastart == (uintptr_t)MAP_FAILED)) {
-		uk_pr_err("Failed to mmap the phdr at offset %lu\n",
-			  phdr->p_offset);
-		return (int)vastart;
+	if (phdr->p_filesz) {
+		vastart = (uintptr_t)mmap(addr, phdr->p_filesz + delta_p_offset,
+					  get_phdr_mmap_prot(phdr),
+					  MAP_FIXED | MAP_PRIVATE,
+					  fd, phdr->p_offset - delta_p_offset);
+		if (unlikely(vastart == (uintptr_t)MAP_FAILED)) {
+			uk_pr_err("Failed to mmap the phdr at offset %lu\n",
+				  phdr->p_offset);
+			return (int)vastart;
+		}
+
+		vastart += phdr->p_filesz + delta_p_offset;
+		vaend = PAGE_ALIGN_UP(vastart +
+				      (phdr->p_memsz - phdr->p_filesz));
+	} else {
+		vastart = (uintptr_t)addr;
+		vaend = PAGE_ALIGN_UP(vastart + phdr->p_memsz +
+				      delta_p_offset);
 	}
 
 	/* mmap anonymously what we are left if memsz > filesz */
-	vastart += phdr->p_filesz + delta_p_offset;
-	vaend = PAGE_ALIGN_UP(vastart + (phdr->p_memsz - phdr->p_filesz));
 	if (vaend > vastart) {
 		rc = elf_load_mmap_filesz_memsz_diff(elf_prog, phdr,
 						     vastart, vaend);