libelf: Return error if elf_compress_gnu is used on SHF_COMPRESSED section.

Compressing a section that is already compressed is fine, but useless.
But it isn't possible to gnu compress (or decompress) a SHF_COMPRESSED
section since there is no state kept that would tell if the section was
first GNU compressed or first gabi compressed. Calling elf_compress_gnu
on a section and then calling elf_compress on it to decompress it twice
could cause a crash (the other way around is fine). Just disallow it.

https://sourceware.org/bugzilla/show_bug.cgi?id=23528

Signed-off-by: Mark Wielaard <mark@klomp.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/libelf/libelf.h b/libelf/libelf.h
index 61f19231..d11358cc 100644
--- a/libelf/libelf.h
+++ b/libelf/libelf.h
@@ -366,6 +366,11 @@ extern Elf64_Chdr *elf64_getchdr (Elf_Scn *__scn);
    It is an error to request compression for a section that already
    has SHF_COMPRESSED set, or (for elf_compress) to request
    decompression for an section that doesn't have SHF_COMPRESSED set.
+   If a section has SHF_COMPRESSED set then calling elf_compress_gnu
+   will result in an error.  The section has to be decompressed first
+   using elf_compress.  Calling elf_compress on a section compressed
+   with elf_compress_gnu is fine, but probably useless.
+
    It is always an error to call these functions on SHT_NOBITS
    sections or if the section has the SHF_ALLOC flag set.
    elf_compress_gnu will not check whether the section name starts