There is a potential problem with resolve_ref() and some other functions in git. The return value returned by resolve_ref() may be changed when the function is called again. Callers must make sure the next call won't happen as long as the value is still being used. It's usually hard to track down this kind of problem. Michael Haggerty has an idea [1] that, instead of passing the same static buffer to caller every time the function is called, we free the old buffer and allocate the new one. This way access to the old (now invalid) buffer may be caught. This patch applies the same principle for resolve_ref() with a few modifications: - This behavior is enabled when GIT_DEBUG_MEMCHECK is set. The ability is always available. We may be able to ask users to rerun with this flag on in suspicious cases. - Rely on mmap/mprotect to catch illegal access. We need valgrind or some other memory tracking tool to reliably catch this in Michael's approach. - Because mprotect is used instead of munmap, we definitely leak memory. Hopefully callers will not put resolve_ref() in a loop that runs 1 million times. - Save caller location in the allocated buffer so we know who made this call in the core dump. Also introduce a new target, "make memcheck", that runs tests with this flag on. [1] http://comments.gmane.org/gmane.comp.version-control.git/182209 Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx> --- Changes include: - __FUNCTION__ to __FILE__ for compiler compatibility - x{malloc,free}_mmap() now put call site information in a struct, it's clearer this way and hopefully will avoid platform issues - update t0071 to follow '&&' convention - add notes where to get caller site info in git-compat-util.h .gitignore | 1 + Makefile | 4 ++++ cache.h | 3 ++- git-compat-util.h | 20 ++++++++++++++++++++ refs.c | 13 +++++++++++-- t/t0071-memcheck.sh | 11 +++++++++++ test-resolve-ref.c | 18 ++++++++++++++++++ wrapper.c | 27 +++++++++++++++++++++++++++ 8 files changed, 94 insertions(+), 3 deletions(-) create mode 100755 t/t0071-memcheck.sh create mode 100644 test-resolve-ref.c diff --git a/.gitignore b/.gitignore index 8572c8c..470e452 100644 --- a/.gitignore +++ b/.gitignore @@ -179,6 +179,7 @@ /test-obj-pool /test-parse-options /test-path-utils +/test-resolve-ref /test-run-command /test-sha1 /test-sigchain diff --git a/Makefile b/Makefile index ed82320..d71cf04 100644 --- a/Makefile +++ b/Makefile @@ -444,6 +444,7 @@ TEST_PROGRAMS_NEED_X += test-obj-pool TEST_PROGRAMS_NEED_X += test-parse-options TEST_PROGRAMS_NEED_X += test-path-utils TEST_PROGRAMS_NEED_X += test-run-command +TEST_PROGRAMS_NEED_X += test-resolve-ref TEST_PROGRAMS_NEED_X += test-sha1 TEST_PROGRAMS_NEED_X += test-sigchain TEST_PROGRAMS_NEED_X += test-string-pool @@ -2241,6 +2242,9 @@ export NO_SVN_TESTS test: all $(MAKE) -C t/ all +memcheck: all + GIT_DEBUG_MEMCHECK=1 $(MAKE) -C t/ all + test-ctype$X: ctype.o test-date$X: date.o ctype.o diff --git a/cache.h b/cache.h index 4887a3e..ba5e911 100644 --- a/cache.h +++ b/cache.h @@ -865,7 +865,8 @@ extern int read_ref(const char *filename, unsigned char *sha1); * * errno is sometimes set on errors, but not always. */ -extern const char *resolve_ref(const char *ref, unsigned char *sha1, int reading, int *flag); +#define resolve_ref(ref, sha1, reading, flag) resolve_ref_real(ref, sha1, reading, flag, __FILE__, __LINE__) +extern const char *resolve_ref_real(const char *ref, unsigned char *sha1, int reading, int *flag, const char *file, int line); extern char *resolve_refdup(const char *ref, unsigned char *sha1, int reading, int *flag); extern int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref); diff --git a/git-compat-util.h b/git-compat-util.h index 77062ed..0cb6e34 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -433,6 +433,26 @@ extern char *xstrndup(const char *str, size_t len); extern void *xrealloc(void *ptr, size_t size); extern void *xcalloc(size_t nmemb, size_t size); extern void *xmmap(void *start, size_t length, int prot, int flags, int fd, off_t offset); + +/* + * These functions are used to allocate new memory blocks and catch + * invalid use after they are released (though the memory is never + * returned to system, so do not allocate too much this way). + * + * mprotect() is used to remove all access to memory when xfree_mmap() + * is called. Invalid access will cause sigsegv. The memory block is + * preceded by struct alloc_header, describing where it is + * allocated. This information can be found in the core dump. + */ +struct alloc_header { + const char *file; + int line; + int size; + char buf[FLEX_ARRAY]; +}; +extern void *xmalloc_mmap(size_t, const char *file, int line); +extern void xfree_mmap(void *); + extern ssize_t xread(int fd, void *buf, size_t len); extern ssize_t xwrite(int fd, const void *buf, size_t len); extern int xdup(int fd); diff --git a/refs.c b/refs.c index 8ffb32f..cf8dfcc 100644 --- a/refs.c +++ b/refs.c @@ -497,12 +497,21 @@ static int get_packed_ref(const char *ref, unsigned char *sha1) return -1; } -const char *resolve_ref(const char *ref, unsigned char *sha1, int reading, int *flag) +const char *resolve_ref_real(const char *ref, unsigned char *sha1, + int reading, int *flag, const char *file, int line) { int depth = MAXDEPTH; ssize_t len; char buffer[256]; - static char ref_buffer[256]; + static char real_ref_buffer[256]; + static char *ref_buffer; + + if (!ref_buffer && !getenv("GIT_DEBUG_MEMCHECK")) + ref_buffer = real_ref_buffer; + if (ref_buffer != real_ref_buffer) { + xfree_mmap(ref_buffer); + ref_buffer = xmalloc_mmap(256, file, line); + } if (flag) *flag = 0; diff --git a/t/t0071-memcheck.sh b/t/t0071-memcheck.sh new file mode 100755 index 0000000..8904369 --- /dev/null +++ b/t/t0071-memcheck.sh @@ -0,0 +1,11 @@ +#!/bin/sh + +test_description='test that GIT_DEBUG_MEMCHECK works correctly' +. ./test-lib.sh + +test_expect_success 'test-resolve-ref must crash' ' + test-resolve-ref && + GIT_DEBUG_MEMCHECK=1 test_expect_code 139 test-resolve-ref +' + +test_done diff --git a/test-resolve-ref.c b/test-resolve-ref.c new file mode 100644 index 0000000..b772038 --- /dev/null +++ b/test-resolve-ref.c @@ -0,0 +1,18 @@ +#include "cache.h" + +int main(int argc, char **argv) +{ + unsigned char sha1[20]; + const char *ref1, *ref2; + setup_git_directory(); + + /* + * This is an invalid use of resolve_ref_unsafe(). This + * function returns a shared buffer, so by the time the second + * call is made, ref1 must _not_ be accessed any more. + */ + ref1 = resolve_ref("HEAD", sha1, 0, NULL); + ref2 = resolve_ref("HEAD", sha1, 0, NULL); + printf("ref1 = %s\nref2 = %s\n", ref1, ref2); + return 0; +} diff --git a/wrapper.c b/wrapper.c index 85f09df..02b6c81 100644 --- a/wrapper.c +++ b/wrapper.c @@ -60,6 +60,33 @@ void *xmallocz(size_t size) return ret; } +void *xmalloc_mmap(size_t size, const char *file, int line) +{ + struct alloc_header *block; + size += offsetof(struct alloc_header,buf); + block = mmap(NULL, size, PROT_READ | PROT_WRITE, + MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); + if (block == (struct alloc_header*)-1) + die_errno("unable to mmap %lu bytes anonymously", + (unsigned long)size); + + block->file = file; + block->line = line; + block->size = size; + return block->buf; +} + +void xfree_mmap(void *p) +{ + struct alloc_header *block; + + if (!p) + return; + block = (struct alloc_header *)((char*)p - offsetof(struct alloc_header,buf)); + if (mprotect(block, block->size, PROT_NONE) == -1) + die_errno("unable to remove memory access"); +} + /* * xmemdupz() allocates (len + 1) bytes of memory, duplicates "len" bytes of * "data" to the allocated memory, zero terminates the allocated memory, -- 1.7.8.36.g69ee2 -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html