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Avoid using large buffers in 'string.format'

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The result of "string.format("%.99f", -1e308) is 410 characters long,
but all other formats have much smaller limits (at most 99 plus a fex
extras). This commit avoids 'string.format' asking for a buffer
~400 chars large when ~100 will do.
This commit is contained in:
Roberto Ierusalimschy 2019-04-12 11:48:24 -03:00
parent b0810c51c3
commit 2d3f095448
2 changed files with 29 additions and 20 deletions
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40
lstrlib.c
View File

@ -1038,13 +1038,23 @@ static int lua_number2strx (lua_State *L, char *buff, int sz,
/*
** Maximum size of each formatted item. This maximum size is produced
** Maximum size for items formatted with '%f'. This size is produced
** by format('%.99f', -maxfloat), and is equal to 99 + 3 ('-', '.',
** and '\0') + number of decimal digits to represent maxfloat (which
** is maximum exponent + 1). (99+3+1 then rounded to 120 for "extra
** expenses", such as locale-dependent stuff)
** is maximum exponent + 1). (99+3+1, adding some extra, 110)
*/
#define MAX_ITEM (120 + l_mathlim(MAX_10_EXP))
#define MAX_ITEMF (110 + l_mathlim(MAX_10_EXP))
/*
** All formats except '%f' do not need that large limit. The other
** float formats use exponents, so that they fit in the 99 limit for
** significant digits; 's' for large strings and 'q' add items directly
** to the buffer; all integer formats also fit in the 99 limit. The
** worst case are floats: they may need 99 significant digits, plus
** '0x', '-', '.', 'e+XXXX', and '\0'. Adding some extra, 120.
*/
#define MAX_ITEM 120
/* valid flags in a format specification */
@ -1194,38 +1204,44 @@ static int str_format (lua_State *L) {
luaL_addchar(&b, *strfrmt++); /* %% */
else { /* format item */
char form[MAX_FORMAT]; /* to store the format ('%...') */
char *buff = luaL_prepbuffsize(&b, MAX_ITEM); /* to put formatted item */
int maxitem = MAX_ITEM;
char *buff = luaL_prepbuffsize(&b, maxitem); /* to put formatted item */
int nb = 0; /* number of bytes in added item */
if (++arg > top)
return luaL_argerror(L, arg, "no value");
strfrmt = scanformat(L, strfrmt, form);
switch (*strfrmt++) {
case 'c': {
nb = l_sprintf(buff, MAX_ITEM, form, (int)luaL_checkinteger(L, arg));
nb = l_sprintf(buff, maxitem, form, (int)luaL_checkinteger(L, arg));
break;
}
case 'd': case 'i':
case 'o': case 'u': case 'x': case 'X': {
lua_Integer n = luaL_checkinteger(L, arg);
addlenmod(form, LUA_INTEGER_FRMLEN);
nb = l_sprintf(buff, MAX_ITEM, form, (LUAI_UACINT)n);
nb = l_sprintf(buff, maxitem, form, (LUAI_UACINT)n);
break;
}
case 'a': case 'A':
addlenmod(form, LUA_NUMBER_FRMLEN);
nb = lua_number2strx(L, buff, MAX_ITEM, form,
nb = lua_number2strx(L, buff, maxitem, form,
luaL_checknumber(L, arg));
break;
case 'e': case 'E': case 'f':
case 'g': case 'G': {
lua_Number n = luaL_checknumber(L, arg);
if (*(strfrmt - 1) == 'f' && l_mathop(fabs)(n) >= 1e100) {
/* 'n' needs more than 99 digits */
maxitem = MAX_ITEMF; /* extra space for '%f' */
buff = luaL_prepbuffsize(&b, maxitem);
}
addlenmod(form, LUA_NUMBER_FRMLEN);
nb = l_sprintf(buff, MAX_ITEM, form, (LUAI_UACNUMBER)n);
nb = l_sprintf(buff, maxitem, form, (LUAI_UACNUMBER)n);
break;
}
case 'p': {
const void *p = lua_topointer(L, arg);
nb = l_sprintf(buff, MAX_ITEM, form, p);
nb = l_sprintf(buff, maxitem, form, p);
break;
}
case 'q': {
@ -1246,7 +1262,7 @@ static int str_format (lua_State *L) {
luaL_addvalue(&b); /* keep entire string */
}
else { /* format the string into 'buff' */
nb = l_sprintf(buff, MAX_ITEM, form, s);
nb = l_sprintf(buff, maxitem, form, s);
lua_pop(L, 1); /* remove result from 'luaL_tolstring' */
}
}
@ -1256,7 +1272,7 @@ static int str_format (lua_State *L) {
return luaL_error(L, "invalid conversion '%s' to 'format'", form);
}
}
lua_assert(nb < MAX_ITEM);
lua_assert(nb < maxitem);
luaL_addsize(&b, nb);
}
}

9
luaconf.h
View File

@ -709,16 +709,9 @@
/*
@@ LUAL_BUFFERSIZE is the buffer size used by the lauxlib buffer system.
** CHANGE it if it uses too much C-stack space. (For long double,
** 'string.format("%.99f", -1e4932)' needs 5052 bytes, so a
** smaller buffer would force a memory allocation for each call to
** 'string.format'.)
*/
#if LUA_FLOAT_TYPE == LUA_FLOAT_LONGDOUBLE
#define LUAL_BUFFERSIZE 8192
#else
#define LUAL_BUFFERSIZE ((int)(16 * sizeof(void*) * sizeof(lua_Number)))
#endif
/*
@@ LUAI_MAXALIGN defines fields that, when used in a union, ensure