From 13f5f57e0d2fb3e06e15c57d67a40499a5910ba6 Mon Sep 17 00:00:00 2001 From: Andrew Burgess Date: Wed, 31 May 2023 21:41:48 +0100 Subject: [PATCH] gdb: check max-value-size when reading strings for printf I noticed that the printf code for strings, printf_c_string and printf_wide_c_string, don't take max-value-size into account, but do load a complete string from the inferior into a GDB buffer. As such it would be possible for an badly behaved inferior to cause GDB to try and allocate an excessively large buffer, potentially crashing GDB, or at least causing GDB to swap lots, which isn't great. We already have a setting to protect against this sort of thing, the 'max-value-size'. So this commit updates the two function mentioned above to check the max-value-size and give an error if the max-value-size is exceeded. If the max-value-size is exceeded, I chose to continue reading inferior memory to figure out how long the string actually is, we just don't store the results. The benefit of this is that when we give the user an error we can tell the user how big the string actually is, which would allow them to correctly adjust max-value-size, if that's what they choose to do. The default for max-value-size is 64k so there should be no user visible changes after this commit, unless the user was previously printing very large strings. If that is the case then the user will now need to increase max-value-size. --- gdb/NEWS | 6 ++++ gdb/printcmd.c | 39 ++++++++++++++++++++--- gdb/testsuite/gdb.base/printcmds.c | 2 ++ gdb/testsuite/gdb.base/printcmds.exp | 5 +++ gdb/testsuite/gdb.base/printf-wchar_t.c | 2 ++ gdb/testsuite/gdb.base/printf-wchar_t.exp | 6 ++++ gdb/testsuite/lib/gdb.exp | 30 +++++++++++++++++ gdb/value.c | 10 +++++- gdb/value.h | 5 +++ 9 files changed, 100 insertions(+), 5 deletions(-) diff --git a/gdb/NEWS b/gdb/NEWS index fd42864c692..b924834d3d7 100644 --- a/gdb/NEWS +++ b/gdb/NEWS @@ -78,6 +78,12 @@ functionality is also available for dprintf when dprintf-style is 'gdb'. +* When the printf command requires a string to be fetched from the + inferior, GDB now uses the existing 'max-value-size' setting to the + limit the memory allocated within GDB. The default 'max-value-size' + is 64k. To print longer strings you should increase + 'max-value-size'. + * New commands maintenance print record-instruction [ N ] diff --git a/gdb/printcmd.c b/gdb/printcmd.c index 0535128c63f..8d7d04231fe 100644 --- a/gdb/printcmd.c +++ b/gdb/printcmd.c @@ -2480,17 +2480,24 @@ printf_c_string (struct ui_file *stream, const char *format, /* This is a %s argument. Build the string in STR which is currently empty. */ gdb_assert (str.size () == 0); - for (size_t len = 0;; len++) + size_t len; + for (len = 0;; len++) { gdb_byte c; QUIT; + read_memory (tem + len, &c, 1); - str.push_back (c); + if (!exceeds_max_value_size (len + 1)) + str.push_back (c); if (c == 0) break; } + if (exceeds_max_value_size (len + 1)) + error (_("printed string requires %s bytes, which is more than " + "max-value-size"), plongest (len + 1)); + /* We will have passed through the above loop at least once, and will only exit the loop when we have pushed a zero byte onto the end of STR. */ @@ -2547,13 +2554,37 @@ printf_wide_c_string (struct ui_file *stream, const char *format, for (len = 0;; len += wcwidth) { QUIT; - tem_str->resize (tem_str->size () + wcwidth); - gdb_byte *dst = tem_str->data () + len; + gdb_byte *dst; + if (!exceeds_max_value_size (len + wcwidth)) + { + tem_str->resize (tem_str->size () + wcwidth); + dst = tem_str->data () + len; + } + else + { + /* We still need to check for the null-character, so we need + somewhere to place the data read from the inferior. We + can't keep growing TEM_STR, it's gotten too big, so + instead just read the new character into the start of + TEMS_STR. This will corrupt the previously read contents, + but we're not going to print this string anyway, we just + want to know how big it would have been so we can tell the + user in the error message (see below). + + And we know there will be space in this buffer so long as + WCWIDTH is smaller than our LONGEST type, the + max-value-size can't be smaller than a LONGEST. */ + dst = tem_str->data (); + } read_memory (tem + len, dst, wcwidth); if (extract_unsigned_integer (dst, wcwidth, byte_order) == 0) break; } + if (exceeds_max_value_size (len + wcwidth)) + error (_("printed string requires %s bytes, which is more than " + "max-value-size"), plongest (len + wcwidth)); + str = tem_str->data (); } diff --git a/gdb/testsuite/gdb.base/printcmds.c b/gdb/testsuite/gdb.base/printcmds.c index fa3a62d6cdd..8445fcc1aa2 100644 --- a/gdb/testsuite/gdb.base/printcmds.c +++ b/gdb/testsuite/gdb.base/printcmds.c @@ -75,6 +75,8 @@ char *teststring = (char*)"teststring contents"; typedef char *charptr; charptr teststring2 = "more contents"; +const char *teststring3 = "this is a longer test string that we can use"; + /* Test printing of a struct containing character arrays. */ struct some_arrays { diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp index 73f145c9586..eab0264af63 100644 --- a/gdb/testsuite/gdb.base/printcmds.exp +++ b/gdb/testsuite/gdb.base/printcmds.exp @@ -901,6 +901,11 @@ proc test_printf {} { # PR cli/14977. gdb_test "printf \"%s\\n\", 0" "\\(null\\)" + + with_max_value_size 20 { + gdb_test {printf "%s", teststring3} \ + "^printed string requires 45 bytes, which is more than max-value-size" + } } #Test printing DFP values with printf diff --git a/gdb/testsuite/gdb.base/printf-wchar_t.c b/gdb/testsuite/gdb.base/printf-wchar_t.c index 4d13fd3c961..2635932c780 100644 --- a/gdb/testsuite/gdb.base/printf-wchar_t.c +++ b/gdb/testsuite/gdb.base/printf-wchar_t.c @@ -18,6 +18,8 @@ #include const wchar_t wide_str[] = L"wide string"; +const wchar_t long_wide_str[] + = L"this is a much longer wide string that we can use if needed"; int main (void) diff --git a/gdb/testsuite/gdb.base/printf-wchar_t.exp b/gdb/testsuite/gdb.base/printf-wchar_t.exp index 85c6edf292c..8a6003d5785 100644 --- a/gdb/testsuite/gdb.base/printf-wchar_t.exp +++ b/gdb/testsuite/gdb.base/printf-wchar_t.exp @@ -24,3 +24,9 @@ if {![runto_main]} { } gdb_test {printf "%ls\n", wide_str} "^wide string" + +# Check that if the max-value-size will kick in when using printf on strings. +with_max_value_size 20 { + gdb_test {printf "%ls\n", long_wide_str} \ + "^printed string requires 240 bytes, which is more than max-value-size" +} diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp index b4900ae25a6..bf8cdbd750c 100644 --- a/gdb/testsuite/lib/gdb.exp +++ b/gdb/testsuite/lib/gdb.exp @@ -3192,6 +3192,36 @@ proc with_target_charset { target_charset body } { } } +# Run tests in BODY with max-value-size set to SIZE. When BODY is +# finished restore max-value-size. + +proc with_max_value_size { size body } { + global gdb_prompt + + set saved "" + gdb_test_multiple "show max-value-size" "" { + -re -wrap "Maximum value size is ($::decimal) bytes\\." { + set saved $expect_out(1,string) + } + -re ".*$gdb_prompt " { + fail "get max-value-size" + } + } + + gdb_test_no_output -nopass "set max-value-size $size" + + set code [catch {uplevel 1 $body} result] + + gdb_test_no_output -nopass "set max-value-size $saved" + + if {$code == 1} { + global errorInfo errorCode + return -code $code -errorinfo $errorInfo -errorcode $errorCode $result + } else { + return -code $code $result + } +} + # Switch the default spawn id to SPAWN_ID, so that gdb_test, # mi_gdb_test etc. default to using it. diff --git a/gdb/value.c b/gdb/value.c index 1b31f91b4dc..4226e793ac9 100644 --- a/gdb/value.c +++ b/gdb/value.c @@ -804,7 +804,7 @@ check_type_length_before_alloc (const struct type *type) { ULONGEST length = type->length (); - if (max_value_size > -1 && length > max_value_size) + if (exceeds_max_value_size (length)) { if (type->name () != NULL) error (_("value of type `%s' requires %s bytes, which is more " @@ -815,6 +815,14 @@ check_type_length_before_alloc (const struct type *type) } } +/* See value.h. */ + +bool +exceeds_max_value_size (ULONGEST length) +{ + return max_value_size > -1 && length > max_value_size; +} + /* When this has a value, it is used to limit the number of array elements of an array that are loaded into memory when an array value is made non-lazy. */ diff --git a/gdb/value.h b/gdb/value.h index 512d05eac1b..e5c63dccbe2 100644 --- a/gdb/value.h +++ b/gdb/value.h @@ -1612,6 +1612,11 @@ extern void finalize_values (); of floating-point, fixed-point, or integer type. */ extern gdb_mpq value_to_gdb_mpq (struct value *value); +/* Return true if LEN (in bytes) exceeds the max-value-size setting, + otherwise, return false. If the user has disabled (set to unlimited) + the max-value-size setting then this function will always return false. */ +extern bool exceeds_max_value_size (ULONGEST length); + /* While an instance of this class is live, and array values that are created, that are larger than max_value_size, will be restricted in size to a particular number of elements. */ -- 2.30.2