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content_encoding: use writer struct subclasses for different encodings

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The variable-sized encoding-specific storage of a struct contenc_writer
currently relies on void * alignment that may be insufficient with
regards to the specific storage fields, although having not caused any
problems yet.

In addition, gcc 11.3 issues a warning on access to fields of partially
allocated structures that can occur when the specific storage size is 0:

  content_encoding.c: In function ‘Curl_build_unencoding_stack’:
  content_encoding.c:980:21: warning: array subscript ‘struct contenc_writer[0]’ is partly outside array bounds of ‘unsigned char[16]’ [-Warray-bounds]
    980 |     writer->handler = handler;
        |     ~~~~~~~~~~~~~~~~^~~~~~~~~
  In file included from content_encoding.c:49:
  memdebug.h:115:29: note: referencing an object of size 16 allocated by ‘curl_dbg_calloc’
    115 | #define calloc(nbelem,size) curl_dbg_calloc(nbelem, size, __LINE__, __FILE__)
        |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  content_encoding.c:977:60: note: in expansion of macro ‘calloc’
    977 |   struct contenc_writer *writer = (struct contenc_writer *)calloc(1, sz);

To solve both these problems, the current commit replaces the
contenc_writer/params structure pairs by "subclasses" of struct
contenc_writer. These are structures that contain a contenc_writer at
offset 0. Proper field alignment is therefore handled by the compiler and
full structure allocation is performed, silencing the warnings.

Closes #9455
This commit is contained in:
Patrick Monnerat 2022-09-09 13:25:02 +02:00 • committed by Daniel Stenberg
parent ae4d1437f1
commit 4399b0303a
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GPG key ID: 5CC908FDB71E12C2
2 changed files with 38 additions and 31 deletions
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3
lib/content_encoding.h
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@ -28,7 +28,6 @@
struct contenc_writer {
const struct content_encoding *handler; /* Encoding handler. */
struct contenc_writer *downstream; /* Downstream writer. */
void *params; /* Encoding-specific storage (variable length). */
};
/* Content encoding writer. */
@ -42,7 +41,7 @@ struct content_encoding {
const char *buf, size_t nbytes);
void (*close_writer)(struct Curl_easy *data,
struct contenc_writer *writer);
size_t paramsize;
size_t writersize;
};