[PATCH v2 0/2] range-diff: add configurable memory limit for cost matrix

["Paulo Casaretto via GitGitGadget" <gitgitgadget@gmail.com>]

Problem Description
===================

When git range-diff is given extremely large ranges, it can result in
either:

 1. Segmentation fault due to integer overflow in array index calculations
 2. Excessive memory consumption leading to system hangs or OOM kills
 3. Poor user experience with the command appearing to hang for minutes


Reproduction Case
=================

In a Shopify's large monorepo a range-diff command like this crashes after
several minutes with a SIGBUS error

$ git range-diff 4430f36511..316c1276c6 cb5240b6a8..2bbd292091


Range statistics:

 * First range: 256,783 commits
 * Second range: 1 commit
 * Total: 256,784 commits
 * Memory required for cost matrix: n² × 4 bytes = ~260GB


Stack Trace (Segmentation Fault)
================================

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x6e000ae3b8)
  * frame #0: 0x000000010029a284 git`get_correspondences(a=0x000000016fde6188, b=0x000000016fde6160, creation_factor=60) at range-diff.c:356:20
    frame #1: 0x0000000100299310 git`show_range_diff(range1="4430f36511cbacf5c517c6851e2b8508a72dfd30..316c1276c63f55ad9413fa18bf3b6483564a9cf4", range2="cb5240b6a8ba59b4a1f282559ee0742721b0cafc..2bbd292091e376d177ce62e264dae8872ca6be5a", range_diff_opts=0x000000016fde6308) at range-diff.c:593:3
    frame #2: 0x00000001000c719c git`cmd_range_diff(argc=2, argv=0x0000600000bcd8c0, prefix="areas/core/shopify/", repo=0x0000000100468b20) at range-diff.c:167:8
    frame #3: 0x000000010000277c git`run_builtin(p=0x00000001004408d8, argc=3, argv=0x0000600000bcd8c0, repo=0x0000000100468b20) at git.c:480:11
    frame #4: 0x0000000100001020 git`handle_builtin(args=0x000000016fde6c70) at git.c:746:9
    frame #5: 0x0000000100002074 git`run_argv(args=0x000000016fde6c70) at git.c:813:4
    frame #6: 0x0000000100000d3c git`cmd_main(argc=3, argv=0x000000016fde7350) at git.c:953:19
    frame #7: 0x000000010012750c git`main(argc=4, argv=0x000000016fde7348) at common-main.c:9:11
    frame #8: 0x000000018573ab98 dyld`start + 6076



Root Cause Analysis
===================

The crash occurs in get_correspondences() at line 356:

static void get_correspondences(struct string_list *a, struct string_list *b, ...)
{
    int n = a->nr + b->nr;  // Integer overflow: 256,784 fits in int
    ...
    ALLOC_ARRAY(cost, st_mult(n, n));  // Would allocate ~260GB
    ...
    cost[i + n * j] = c;  // Line 356: Invalid memory access
}


Problems:

 1. Integer overflow: While n=256,784 fits in an int, n*n overflows
 2. Memory allocation: Even with proper types, allocating 260GB is
    impractical


Solution
========

Add a memory limit check in get_correspondences() before allocating the cost
matrix. This check uses the total size in bytes (n² × sizeof(int)) and
compares it against a configurable maximum, preventing both excessive memory
usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts
human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit
systems and 2GB for 32 bit systems. This allows comparing ranges of
approximately 32,000 (16,000) commits - generous for real-world use cases
while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message
showing both the requested memory size and the maximum allowed, formatted in
human-readable units for better user experience.

Example usage: git range-diff --max-memory=1G branch1...branch2 git
range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:

 * Pre-counting commits: Would require spawning additional git processes and
   reading all commits twice
 * Limiting by commit count: Less precise than actual memory usage
 * Streaming approach: Would require significant refactoring of the current
   algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/RFC-cover-v2-0.5-00000000000-20211210T122901Z-avarab@gmail.com/

Acked-by: Johannes Schindelin johannes.schindelin@gmx.de
[https://github.com/gitgitgadget/git/pull/1958#issuecomment-3224133138]

pcasaretto (2):
  range-diff: reorder options lexicographically
  range-diff: add configurable memory limit for cost matrix

 builtin/log.c        |  1 +
 builtin/range-diff.c | 29 +++++++++++++++++++++++++----
 log-tree.c           |  1 +
 range-diff.c         | 20 ++++++++++++++++----
 range-diff.h         |  5 +++++
 5 files changed, 48 insertions(+), 8 deletions(-)


base-commit: 954d33a9757fcfab723a824116902f1eb16e05f7
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1958%2Fpcasaretto%2Frange-diff-size-limit-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1958/pcasaretto/range-diff-size-limit-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1958

Range-diff vs v1:

 -:  ----------- > 1:  ec5dcdf9d00 range-diff: reorder options lexicographically
 1:  5cf3e8921a7 ! 2:  c81f920fee0 range-diff: add configurable memory limit for cost matrix
     @@ Commit message
          This issue was previously discussed in:
          https://lore.kernel.org/git/RFC-cover-v2-0.5-00000000000-20211210T122901Z-avarab@gmail.com/
      
     -    Acked-by: Johannes Schindelin johannes.schindelin@gmx.de
     -    Signed-off-by: pcasaretto <paulo.casaretto@shopify.com>
     +    Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
     +    Signed-off-by: Paulo Casaretto <paulo.casaretto@shopify.com>
      
       ## builtin/log.c ##
      @@ builtin/log.c: static void make_cover_letter(struct rev_info *rev, int use_separate_file,
     @@ builtin/range-diff.c: N_("git range-diff [<options>] <base> <old-tip> <new-tip>"
      +	size_t *max_memory = opt->value;
      +	uintmax_t val;
      +
     -+	if (unset) {
     ++	if (unset)
      +		return 0;
     -+	}
      +
      +	if (!git_parse_unsigned(arg, &val, SIZE_MAX))
      +		return error(_("invalid max-memory value: %s"), arg);
     @@ builtin/range-diff.c: int cmd_range_diff(int argc,
       		.other_arg = &other_arg
       	};
      @@ builtin/range-diff.c: int cmd_range_diff(int argc,
     - 		OPT_INTEGER(0, "creation-factor",
     - 			    &range_diff_opts.creation_factor,
     - 			    N_("percentage by which creation is weighted")),
     -+		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
     -+				  N_("style"), N_("passed to 'git log'"), 0),
     -+		OPT_BOOL(0, "left-only", &left_only,
     -+			 N_("only emit output related to the first range")),
     + 				  N_("style"), N_("passed to 'git log'"), 0),
     + 		OPT_BOOL(0, "left-only", &left_only,
     + 			 N_("only emit output related to the first range")),
      +		OPT_CALLBACK(0, "max-memory", &range_diff_opts.max_memory,
      +			     N_("size"),
      +			     N_("maximum memory for cost matrix (default 4G)"),
     @@ builtin/range-diff.c: int cmd_range_diff(int argc,
       		OPT_BOOL(0, "no-dual-color", &simple_color,
       			    N_("use simple diff colors")),
       		OPT_PASSTHRU_ARGV(0, "notes", &other_arg,
     - 				  N_("notes"), N_("passed to 'git log'"),
     - 				  PARSE_OPT_OPTARG),
     --		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
     --				  N_("style"), N_("passed to 'git log'"), 0),
     - 		OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
     - 				  N_("passed to 'git log'"), PARSE_OPT_NOARG),
     --		OPT_BOOL(0, "left-only", &left_only,
     --			 N_("only emit output related to the first range")),
     - 		OPT_BOOL(0, "right-only", &right_only,
     - 			 N_("only emit output related to the second range")),
     - 		OPT_END()
      
       ## log-tree.c ##
      @@ log-tree.c: static void show_diff_of_diff(struct rev_info *opt)
     @@ log-tree.c: static void show_diff_of_diff(struct rev_info *opt)
       
      
       ## range-diff.c ##
     -@@
     - #include "apply.h"
     - #include "revision.h"
     - 
     -+
     - struct patch_util {
     - 	/* For the search for an exact match */
     - 	struct hashmap_entry e;
     -@@ range-diff.c: static void find_exact_matches(struct string_list *a, struct string_list *b)
     - }
     - 
     - static int diffsize_consume(void *data,
     --			     char *line UNUSED,
     --			     unsigned long len UNUSED)
     -+			    char *line UNUSED,
     -+			    unsigned long len UNUSED)
     - {
     - 	(*(int *)data)++;
     - 	return 0;
      @@ range-diff.c: static int diffsize(const char *a, const char *b)
       }
       
     @@ range-diff.c: static int diffsize(const char *a, const char *b)
       	ALLOC_ARRAY(a2b, n);
       	ALLOC_ARRAY(b2a, n);
       
     -@@ range-diff.c: static void get_correspondences(struct string_list *a, struct string_list *b,
     - 		}
     - 
     - 		c = a_util->matching < 0 ?
     --			a_util->diffsize * creation_factor / 100 : COST_MAX;
     -+			    a_util->diffsize * creation_factor / 100 :
     -+			    COST_MAX;
     - 		for (j = b->nr; j < n; j++)
     - 			cost[i + n * j] = c;
     - 	}
     -@@ range-diff.c: static void get_correspondences(struct string_list *a, struct string_list *b,
     - 		struct patch_util *util = b->items[j].util;
     - 
     - 		c = util->matching < 0 ?
     --			util->diffsize * creation_factor / 100 : COST_MAX;
     -+			    util->diffsize * creation_factor / 100 :
     -+			    COST_MAX;
     - 		for (i = a->nr; i < n; i++)
     - 			cost[i + n * j] = c;
     - 	}
     -@@ range-diff.c: static void output(struct string_list *a, struct string_list *b,
     - 		if (i < a->nr && a_util->matching < 0) {
     - 			if (!range_diff_opts->right_only)
     - 				output_pair_header(&opts, patch_no_width,
     --					   &buf, &dashes, a_util, NULL);
     -+						   &buf, &dashes, a_util, NULL);
     - 			i++;
     - 			continue;
     - 		}
     -@@ range-diff.c: static void output(struct string_list *a, struct string_list *b,
     - 		while (j < b->nr && b_util->matching < 0) {
     - 			if (!range_diff_opts->left_only)
     - 				output_pair_header(&opts, patch_no_width,
     --					   &buf, &dashes, NULL, b_util);
     -+						   &buf, &dashes, NULL, b_util);
     - 			b_util = ++j < b->nr ? b->items[j].util : NULL;
     - 		}
     - 
      @@ range-diff.c: int show_range_diff(const char *range1, const char *range2,
       	if (!res) {
       		find_exact_matches(&branch1, &branch2);

-- 
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