102 lines
3.7 KiB
Diff
102 lines
3.7 KiB
Diff
From 2a22de439ec63da1927b640eda309296a1e8dce5 Mon Sep 17 00:00:00 2001
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From: Lasse Collin <lasse.collin@tukaani.org>
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Date: Mon, 13 May 2019 20:05:17 +0300
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Subject: [PATCH 14/14] liblzma: Avoid memcpy(NULL, foo, 0) because it is
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undefined behavior.
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I should have always known this but I didn't. Here is an example
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as a reminder to myself:
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int mycopy(void *dest, void *src, size_t n)
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{
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memcpy(dest, src, n);
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return dest == NULL;
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}
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In the example, a compiler may assume that dest != NULL because
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passing NULL to memcpy() would be undefined behavior. Testing
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with GCC 8.2.1, mycopy(NULL, NULL, 0) returns 1 with -O0 and -O1.
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With -O2 the return value is 0 because the compiler infers that
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dest cannot be NULL because it was already used with memcpy()
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and thus the test for NULL gets optimized out.
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In liblzma, if a null-pointer was passed to memcpy(), there were
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no checks for NULL *after* the memcpy() call, so I cautiously
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suspect that it shouldn't have caused bad behavior in practice,
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but it's hard to be sure, and the problematic cases had to be
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fixed anyway.
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Thanks to Jeffrey Walton.
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---
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src/liblzma/common/common.c | 6 +++++-
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src/liblzma/lz/lz_decoder.c | 12 +++++++++---
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src/liblzma/simple/simple_coder.c | 10 +++++++++-
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3 files changed, 23 insertions(+), 5 deletions(-)
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diff --git a/src/liblzma/common/common.c b/src/liblzma/common/common.c
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index 1399b92..18453ae 100644
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--- a/src/liblzma/common/common.c
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+++ b/src/liblzma/common/common.c
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@@ -99,7 +99,11 @@ lzma_bufcpy(const uint8_t *restrict in, size_t *restrict in_pos,
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const size_t out_avail = out_size - *out_pos;
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const size_t copy_size = my_min(in_avail, out_avail);
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- memcpy(out + *out_pos, in + *in_pos, copy_size);
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+ // Call memcpy() only if there is something to copy. If there is
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+ // nothing to copy, in or out might be NULL and then the memcpy()
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+ // call would trigger undefined behavior.
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+ if (copy_size > 0)
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+ memcpy(out + *out_pos, in + *in_pos, copy_size);
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*in_pos += copy_size;
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*out_pos += copy_size;
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diff --git a/src/liblzma/lz/lz_decoder.c b/src/liblzma/lz/lz_decoder.c
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index bb21d0d..6c9024e 100644
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--- a/src/liblzma/lz/lz_decoder.c
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+++ b/src/liblzma/lz/lz_decoder.c
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@@ -91,11 +91,17 @@ decode_buffer(lzma_coder *coder,
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in, in_pos, in_size);
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// Copy the decoded data from the dictionary to the out[]
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- // buffer.
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+ // buffer. Do it conditionally because out can be NULL
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+ // (in which case copy_size is always 0). Calling memcpy()
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+ // with a null-pointer is undefined even if the third
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+ // argument is 0.
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const size_t copy_size = coder->dict.pos - dict_start;
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assert(copy_size <= out_size - *out_pos);
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- memcpy(out + *out_pos, coder->dict.buf + dict_start,
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- copy_size);
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+
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+ if (copy_size > 0)
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+ memcpy(out + *out_pos, coder->dict.buf + dict_start,
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+ copy_size);
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+
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*out_pos += copy_size;
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// Reset the dictionary if so requested by coder->lz.code().
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diff --git a/src/liblzma/simple/simple_coder.c b/src/liblzma/simple/simple_coder.c
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index 13ebabc..4f499be 100644
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--- a/src/liblzma/simple/simple_coder.c
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+++ b/src/liblzma/simple/simple_coder.c
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@@ -118,7 +118,15 @@ simple_code(void *coder_ptr, const lzma_allocator *allocator,
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// coder->pos and coder->size yet. This way the coder can be
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// restarted if the next filter in the chain returns e.g.
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// LZMA_MEM_ERROR.
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- memcpy(out + *out_pos, coder->buffer + coder->pos, buf_avail);
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+ //
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+ // Do the memcpy() conditionally because out can be NULL
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+ // (in which case buf_avail is always 0). Calling memcpy()
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+ // with a null-pointer is undefined even if the third
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+ // argument is 0.
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+ if (buf_avail > 0)
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+ memcpy(out + *out_pos, coder->buffer + coder->pos,
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+ buf_avail);
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+
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*out_pos += buf_avail;
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// Copy/Encode/Decode more data to out[].
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--
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1.8.3.1
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