libbacktrace patch committed: Don't assume compressed section aligned

libbacktrace patch committed: Don't assume compressed section aligned

[Ian Lance Taylor]

[-- Attachment #1: Type: text/plain, Size: 457 bytes --]

Reportedly when lld compresses debug sections, it fails to set the
alignment of the compressed section such that the compressed header
can be read directly.  To me this seems like a bug in lld.  However,
libbacktrace needs to work around it.  This patch, originally by the
GitHub user ubyte, does that.  Bootstrapped and tested on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian

* elf.c (elf_uncompress_chdr): Don't assume compressed section is
aligned.

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 2562 bytes --]

5825bd0e0d0040126e78269e56c9b9f533e2a520
diff --git a/libbacktrace/elf.c b/libbacktrace/elf.c
index 7841c86cd9c..3cd87020b03 100644
--- a/libbacktrace/elf.c
+++ b/libbacktrace/elf.c
@@ -5076,7 +5076,7 @@ elf_uncompress_chdr (struct backtrace_state *state,
 		     backtrace_error_callback error_callback, void *data,
 		     unsigned char **uncompressed, size_t *uncompressed_size)
 {
-  const b_elf_chdr *chdr;
+  b_elf_chdr chdr;
   char *alc;
   size_t alc_len;
   unsigned char *po;
@@ -5088,27 +5088,30 @@ elf_uncompress_chdr (struct backtrace_state *state,
   if (compressed_size < sizeof (b_elf_chdr))
     return 1;
 
-  chdr = (const b_elf_chdr *) compressed;
+  /* The lld linker can misalign a compressed section, so we can't safely read
+     the fields directly as we can for other ELF sections.  See
+     https://github.com/ianlancetaylor/libbacktrace/pull/120.  */
+  memcpy (&chdr, compressed, sizeof (b_elf_chdr));
 
   alc = NULL;
   alc_len = 0;
-  if (*uncompressed != NULL && *uncompressed_size >= chdr->ch_size)
+  if (*uncompressed != NULL && *uncompressed_size >= chdr.ch_size)
     po = *uncompressed;
   else
     {
-      alc_len = chdr->ch_size;
+      alc_len = chdr.ch_size;
       alc = backtrace_alloc (state, alc_len, error_callback, data);
       if (alc == NULL)
 	return 0;
       po = (unsigned char *) alc;
     }
 
-  switch (chdr->ch_type)
+  switch (chdr.ch_type)
     {
     case ELFCOMPRESS_ZLIB:
       if (!elf_zlib_inflate_and_verify (compressed + sizeof (b_elf_chdr),
 					compressed_size - sizeof (b_elf_chdr),
-					zdebug_table, po, chdr->ch_size))
+					zdebug_table, po, chdr.ch_size))
 	goto skip;
       break;
 
@@ -5116,7 +5119,7 @@ elf_uncompress_chdr (struct backtrace_state *state,
       if (!elf_zstd_decompress (compressed + sizeof (b_elf_chdr),
 				compressed_size - sizeof (b_elf_chdr),
 				(unsigned char *)zdebug_table, po,
-				chdr->ch_size))
+				chdr.ch_size))
 	goto skip;
       break;
 
@@ -5126,7 +5129,7 @@ elf_uncompress_chdr (struct backtrace_state *state,
     }
 
   *uncompressed = po;
-  *uncompressed_size = chdr->ch_size;
+  *uncompressed_size = chdr.ch_size;
 
   return 1;
 
@@ -6876,8 +6879,8 @@ elf_add (struct backtrace_state *state, const char *filename, int descriptor,
 	}
     }
 
-  // A debuginfo file may not have a useful .opd section, but we can use the
-  // one from the original executable.
+  /* A debuginfo file may not have a useful .opd section, but we can use the
+     one from the original executable.  */
   if (opd == NULL)
     opd = caller_opd;
 

Re: libbacktrace patch committed: Don't assume compressed section aligned

[Fangrui Song]

On ELF64, it looks like BFD uses 8-byte alignment for compressed
`.debug_*` sections while gold/lld/mold use 1-byte alignment. I do not
know how the Solaris linker sets the alignment.

The specification's wording makes me confused whether it really
requires 8-byte alignment, even if a non-packed `Elf64_Chdr` surely
requires 8.

> The sh_size and sh_addralign fields of the section header for a compressed section reflect the requirements of the compressed section.

There are many `.debug_*` sections. So avoiding some alignment padding
seems a very natural extension (a DWARF v5 -gsplit-dwarf relocatable
file has ~10 `.debug_*` sections), even if the specification doesn't
allow it with a very strict interpretation...

(Off-topic: I wonder whether ELF control structures should use
unaligned LEB128 more. REL/RELA can naturally be replaced with a
LEB128 one similar to wasm.)

On Fri, Mar 8, 2024 at 1:57 PM Ian Lance Taylor <iant@golang.org> wrote:
>
> Reportedly when lld compresses debug sections, it fails to set the
> alignment of the compressed section such that the compressed header
> can be read directly.  To me this seems like a bug in lld.  However,
> libbacktrace needs to work around it.  This patch, originally by the
> GitHub user ubyte, does that.  Bootstrapped and tested on
> x86_64-pc-linux-gnu.  Committed to mainline.
>
> Ian
>
> * elf.c (elf_uncompress_chdr): Don't assume compressed section is
> aligned.



-- 
宋方睿

Re: libbacktrace patch committed: Don't assume compressed section aligned

[H.J. Lu]

On Fri, Mar 8, 2024 at 2:48 PM Fangrui Song <maskray@google.com> wrote:
>
> On ELF64, it looks like BFD uses 8-byte alignment for compressed
> `.debug_*` sections while gold/lld/mold use 1-byte alignment. I do not
> know how the Solaris linker sets the alignment.
>
> The specification's wording makes me confused whether it really
> requires 8-byte alignment, even if a non-packed `Elf64_Chdr` surely
> requires 8.

Since compressed sections begin with a compression header
structure that identifies the compression algorithm, compressed
sections must be aligned to the alignment of the compression
header.  I don't think there is any ambiguity here.

> > The sh_size and sh_addralign fields of the section header for a compressed section reflect the requirements of the compressed section.
>
> There are many `.debug_*` sections. So avoiding some alignment padding
> seems a very natural extension (a DWARF v5 -gsplit-dwarf relocatable
> file has ~10 `.debug_*` sections), even if the specification doesn't
> allow it with a very strict interpretation...
>
> (Off-topic: I wonder whether ELF control structures should use
> unaligned LEB128 more. REL/RELA can naturally be replaced with a
> LEB128 one similar to wasm.)
>
> On Fri, Mar 8, 2024 at 1:57 PM Ian Lance Taylor <iant@golang.org> wrote:
> >
> > Reportedly when lld compresses debug sections, it fails to set the
> > alignment of the compressed section such that the compressed header
> > can be read directly.  To me this seems like a bug in lld.  However,
> > libbacktrace needs to work around it.  This patch, originally by the
> > GitHub user ubyte, does that.  Bootstrapped and tested on
> > x86_64-pc-linux-gnu.  Committed to mainline.
> >
> > Ian
> >
> > * elf.c (elf_uncompress_chdr): Don't assume compressed section is
> > aligned.
>
>
>
> --
> 宋方睿



-- 
H.J.