Subject: [PATCH v2 0/3] Fix uninitialised reads found with MSAN

From: "Andrzej Hunt via GitGitGadget" <[email-masked]>
Date: Mon, 14 Jun 2021 15:51:13 +0000

V2 replaces an #if'd memset with some brace initialisation (patch 3/3) as
per review comments.

I've also removed an irrelevant "technically" from commit message 2/3, and
fixed a typo in commit message 3/3.

Andrzej Hunt (3):
  bulk-checkin: make buffer reuse more obvious and safer
  split-index: use oideq instead of memcmp to compare object_id's
  builtin/checkout--worker: zero-initialise struct to avoid MSAN
    complaints

 builtin/checkout--worker.c | 2 +-
 bulk-checkin.c             | 3 +--
 split-index.c              | 3 ++-
 3 files changed, 4 insertions(+), 4 deletions(-)

base-commit: 62a8d224e6203d9d3d2d1d63a01cf5647ec312c9
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1033%2Fahunt%2Fmsan-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1033/ahunt/msan-v2
Pull-Request: https://github.com/git/git/pull/1033

Range-diff vs v1:

 1:  7659d4bf13c2 = 1:  7659d4bf13c2 bulk-checkin: make buffer reuse more obvious and safer
 2:  14b0d5dd7fce ! 2:  6943eb511bee split-index: use oideq instead of memcmp to compare object_id's
     @@ Commit message
          include that field when calling memcmp on a subset of the cache_entry.
          Depending on which hashing algorithm is being used, only part of
          object_id.hash is actually being used, therefore including it in a
     -    memcmp() is technically incorrect. Instead we choose to exclude the
     -    object_id when calling memcmp(), and call oideq() separately.
     +    memcmp() is incorrect. Instead we choose to exclude the object_id when
     +    calling memcmp(), and call oideq() separately.
      
          This issue was found when running t1700-split-index with MSAN, see MSAN
          output below (on my machine, offset 76 corresponds to 4 bytes after the
 3:  cd1e1f6985c7 ! 3:  4bdc0b77f6f2 builtin/checkout--worker: memset struct to avoid MSAN complaints
     @@ Metadata
      Author: Andrzej Hunt <[email-masked]>
      
       ## Commit message ##
     -    builtin/checkout--worker: memset struct to avoid MSAN complaints
     +    builtin/checkout--worker: zero-initialise struct to avoid MSAN complaints
      
          report_result() sends a struct to the parent process, but that struct
     -    contains unintialised padding bytes. Running this code under MSAN
     -    rightly triggers a warning - but we also don't care about this warning
     -    because we control the receiving code, and we therefore know that those
     -    padding bytes won't be read on the receiving end. Therefore we add a
     -    memset to convince MSAN that this memory is safe to read - but only
     -    when building with MSAN to avoid this cost in normal usage.
     +    would contain uninitialised padding bytes. Running this code under MSAN
     +    rightly triggers a warning - but we don't particularly care about this
     +    warning because we control the receiving code, and we therefore know
     +    that those padding bytes won't be read on the receiving end.
     +
     +    We could simply suppress this warning under MSAN with the approporiate
     +    ifdef'd attributes, but a less intrusive solution is to 0-initialise the
     +    struct, which guarantees that the padding will also be initialised.
      
          Interestingly, in the error-case branch, we only try to copy the first
          two members of pc_item_result, by copying only PC_ITEM_RESULT_BASE_SIZE
     @@ Commit message
          after the end of the second last member. We could avoid doing this by
          redefining PC_ITEM_RESULT_BASE_SIZE as
          'offsetof(second_last_member) + sizeof(second_last_member)', but there's
     -    no huge benefit to doing so (and our memset hack silences the MSAN
     -    warning in this scenario either way).
     +    no huge benefit to doing so (and this patch silences the MSAN warning in
     +    this scenario either way).
      
          MSAN output from t2080 (partially interleaved due to the
          parallel work :) ):
     @@ Commit message
          Signed-off-by: Andrzej Hunt <[email-masked]>
      
       ## builtin/checkout--worker.c ##
     -@@ builtin/checkout--worker.c: static void report_result(struct parallel_checkout_item *pc_item)
     - 	struct pc_item_result res;
     +@@ builtin/checkout--worker.c: static void packet_to_pc_item(const char *buffer, int len,
     + 
     + static void report_result(struct parallel_checkout_item *pc_item)
     + {
     +-	struct pc_item_result res;
     ++	struct pc_item_result res = { 0 };
       	size_t size;
       
     -+#if defined(__has_feature)
     -+#  if __has_feature(memory_sanitizer)
     -+	// MSAN workaround: res contains padding bytes, which will remain
     -+	// permanently unintialised. Later, we read all of res in order to send
     -+	// it to the parent process - and MSAN (rightly) complains that we're
     -+	// reading those unintialised padding bytes. By memset'ing res we
     -+	// guarantee that there are no uninitialised bytes.
     -+	memset(&res, 0, sizeof(res));
     -+#endif
     -+#endif
     -+
       	res.id = pc_item->id;
     - 	res.status = pc_item->status;
     - 

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