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mmap: use private mappings for read-only files #7315

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Merged
ethomson merged 1 commit into from
Jul 24, 2026
Merged

mmap: use private mappings for read-only files #7315

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2 changes: 1 addition & 1 deletion src/libgit2/indexer.c
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Original file line number Diff line number Diff line change
Expand Up @@ -97,7 +97,7 @@ static int parse_header(struct git_pack_header *hdr, struct git_pack_file *pack)
int error;
git_map map;

if ((error = p_mmap(&map, sizeof(*hdr), GIT_PROT_READ, GIT_MAP_SHARED, pack->mwf.fd, 0)) < 0)
if ((error = p_mmap(&map, sizeof(*hdr), GIT_PROT_READ, GIT_MAP_PRIVATE, pack->mwf.fd, 0)) < 0)
return error;

memcpy(hdr, map.data, sizeof(*hdr));
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2 changes: 1 addition & 1 deletion src/util/futils.c
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Expand Up @@ -359,7 +359,7 @@ int git_futils_mv_withpath(const char *from, const char *to, const mode_t dirmod

int git_futils_mmap_ro(git_map *out, git_file fd, off64_t begin, size_t len)
{
return p_mmap(out, len, GIT_PROT_READ, GIT_MAP_SHARED, fd, begin);
return p_mmap(out, len, GIT_PROT_READ, GIT_MAP_PRIVATE, fd, begin);
}

int git_futils_mmap_ro_file(git_map *out, const char *path)
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28 changes: 28 additions & 0 deletions tests/util/futils.c
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Original file line number Diff line number Diff line change
@@ -1,6 +1,10 @@
#include "clar_libgit2.h"
#include "futils.h"

#if !defined(GIT_WIN32) && !defined(NO_MMAP)
#include <sys/mman.h>
#endif

/* Fixture setup and teardown */
void test_futils__initialize(void)
{
Expand Down Expand Up @@ -37,6 +41,30 @@ void test_futils__writebuffer(void)
git_str_dispose(&append);
}

void test_futils__mmap_ro_uses_map_private(void)
{
#if defined(GIT_WIN32) || defined(NO_MMAP)
cl_skip();
#else
const char content[] = "mapped data";
const char *path = "futils/mmap-file";
git_map map = { 0 };
git_file fd;

cl_git_mkfile(path, content);
cl_assert((fd = p_open(path, O_RDWR)) >= 0);
cl_git_pass(git_futils_mmap_ro(&map, fd, 0, sizeof(content) - 1));

/* A private mapping must not propagate changes back to the file. */
cl_must_pass(mprotect(map.data, map.len, PROT_READ | PROT_WRITE));
((char *)map.data)[0] = 'M';
cl_assert_equal_file(content, sizeof(content) - 1, path);

git_futils_mmap_free(&map);
p_close(fd);
#endif
}

void test_futils__write_hidden_file(void)
{
#ifndef GIT_WIN32
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