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Check for empty buffer when rendering
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Fixes jgm/pandoc#5033.
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For empty documents, `->size` is zero so
`renderer.buffer->ptr[renderer.buffer->size - 1]` will cause an out-of-bounds
read. Empty buffers always point to the global `cmark_strbuf__initbuf` buffer
so we read `cmark_strbuf__initbuf[-1]`.
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The previous work for unbounded memory usage and overflows on the buffer
API had several shortcomings:
1. The total size of the buffer was limited by arbitrarily small
precision on the storage type for buffer indexes (typedef'd as
`bufsize_t`). This is not a good design pattern in secure applications,
particualarly since it requires the addition of helper functions to cast
to/from the native `size` types and the custom type for the buffer, and
check for overflows.
2. The library was calling `abort` on overflow and memory allocation
failures. This is not a good practice for production libraries, since it
turns a potential RCE into a trivial, guaranteed DoS to the whole
application that is linked against the library. It defeats the whole
point of performing overflow or allocation checks when the checks will
crash the library and the enclosing program anyway.
3. The default size limits for buffers were essentially unbounded
(capped to the precision of the storage type) and could lead to DoS
attacks by simple memory exhaustion (particularly critical in 32-bit
platforms). This is not a good practice for a library that handles
arbitrary user input.
Hence, this patchset provides slight (but in my opinion critical)
improvements on this area, copying some of the patterns we've used in
the past for high throughput, security sensitive Markdown parsers:
1. The storage type for buffer sizes is now platform native (`ssize_t`).
Ideally, this would be a `size_t`, but several parts of the code expect
buffer indexes to be possibly negative. Either way, switching to a
`size` type is an strict improvement, particularly in 64-bit platforms.
All the helpers that assured that values cannot escape the `size` range
have been removed, since they are superfluous.
2. The overflow checks have been removed. Instead, the maximum size for
a buffer has been set to a safe value for production usage (32mb) that
can be proven not to overflow in practice. Users that need to parse
particularly large Markdown documents can increase this value. A static,
compile-time check has been added to ensure that the maximum buffer size
cannot overflow on any growth operations.
3. The library no longer aborts on buffer overflow. The CMark library
now follows the convention of other Markdown implementations (such as
Hoedown and Sundown) and silently handles buffer overflows and
allocation failures by dropping data from the buffer. The result is
that pathological Markdown documents that try to exploit the library
will instead generate truncated (but valid, and safe) outputs.
All tests after these small refactorings have been verified to pass.
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NOTE: Regarding 32 bit overflows, generating test cases that crash the
library is trivial (any input document larger than 2gb will crash
CMark), but most Python implementations have issues with large strings
to begin with, so a test case cannot be added to the pathological tests
suite, since it's written in Python.
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We generally want this option to prohibit any breaking
in things like headers (not just wraps, but softbreaks).
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Newer MSVC versions support enough of C99 to be able to compile cmark
in plain C mode. Only the "inline" keyword is still unsupported.
We have to use "__inline" instead.
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We try not to escape punctuation unless we absolutely have
to. So, `)` and `.` are no longer escaped whenever they
occur after digits; now they are only escaped if they are
geuninely in a position where they'd cause a list item.
This required a couple changes to render.c.
- `renderer->begin_content` is only set to false AFTER a
string of digits at the beginning of the line. (This is
slightly unprincipled.)
- We never break before a numeral (also slightly unprincipled).
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This is like `begin_line` except that it doesn't trigger
production of the prefix. So it can be set after an initial
prefix (say `> `) is printed by the renderer, and consulted
in determining whether to escape content that has a special
meaning at the beginning of a line.
Used in the commonmark renderer.
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* Reformatted all source files.
* Added 'format' target to Makefile.
* Removed 'astyle' target.
* Updated .editorconfig.
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Moved begin_line setting into render.c, so you don't need to
worry about it in outc.
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Added options argument to render_node function, and rearrange
argument order.
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Now we just calculate this in the latex renderer.
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This allows us to remove direct manipulation of buffer from
the latex and commonmark renderers.
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* Added functions for cr, blankline, out to renderer object.
* Removed lit (we'll handle this with a macro).
* Changed type of out so it takes a regular string instead of
a chunk.
* Use macros LIT, OUT, BLANKLINE, CR in renderers to simplify code.
(Not sure about this, but `renderer->out(renderer, ...)` sure is
verbose.)
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* Added render.c, render.h.
* Moved common functions and definitions from latex.c and commonmark.c
to render.c, render.h.
* Added a wrapper, cmark_render, that creates a renderer given a
character-escaper and a node renderer.
Closes #63.
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