[llvm-objcopy] Fix crash when writing empty binary output

Summary: When using llvm-objcopy -O binary and the resulting file will be empty (e.g. removing the only section that would be written, or using --only-keep with a section that doesn't exist/isn't SHF_ALLOC), we crash because FileOutputBuffer expects Size > 0. Add a regression test, and change Buffer to open/truncate the output file in this case.

Reviewers: alexshap, jhenderson, jakehehrlich, espindola

Reviewed By: alexshap, jhenderson

Subscribers: jfb, llvm-commits, emaste, arichardson

Differential Revision: https://reviews.llvm.org/D56806

llvm-svn: 352371

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/test/tools/llvm-objcopy/ELF/binary-output-empty.test b/llvm/test/tools/llvm-objcopy/ELF/binary-output-empty.test
new file mode 100644
index 00000000000..d234b58a4a2
--- /dev/null
+++ b/llvm/test/tools/llvm-objcopy/ELF/binary-output-empty.test
@@ -0,0 +1,27 @@
+# RUN: yaml2obj %s -o %t.o
+
+# Writing an empty output to a non-existent file will still create it.
+# RUN: rm -f %t-new.txt
+# RUN: llvm-objcopy -R .text -O binary %t.o %t-new.txt
+# RUN: wc -c %t-new.txt | FileCheck %s
+
+# Writing an empty output to an existing file will truncate it.
+# RUN: echo abcd > %t-existing.txt
+# RUN: llvm-objcopy -R .text -O binary %t.o %t-existing.txt
+# RUN: wc -c %t-existing.txt | FileCheck %s
+
+# In both cases, the file should be empty.
+# CHECK: 0
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_EXEC
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Content:         "c3c3c3c3"
+    Size:            0x1000