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vsprintf: deal with format specifiers with a lookup table

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We did the flags as an array earlier, they had simpler rules.  The final
format specifiers are a bit more complex since they have more fields to
deal with, and we want to handle the length modifiers at the same time.
But like the flags, we're better off just making it a data-driven table
rather than some case statement.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2024-12-19 10:55:56 -08:00
parent 312f48b2e2
commit 614d13462d
1 changed files with 53 additions and 78 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

131
lib/vsprintf.c
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@ -2566,7 +2566,7 @@ static noinline_for_stack
struct fmt format_decode(struct fmt fmt, struct printf_spec *spec) struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
{ {
const char *start = fmt.str; const char *start = fmt.str;
char flag, qualifier; char flag;
/* we finished early by reading the field width */ /* we finished early by reading the field width */
if (fmt.state == FORMAT_STATE_WIDTH) { if (fmt.state == FORMAT_STATE_WIDTH) {
@ -2637,96 +2637,71 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
} }
qualifier: qualifier:
/* get the conversion qualifier */ /* Set up default numeric format */
qualifier = 0;
if (*fmt.str == 'h' || _tolower(*fmt.str) == 'l' ||
*fmt.str == 'z' || *fmt.str == 't') {
qualifier = *fmt.str++;
if (unlikely(qualifier == *fmt.str)) {
if (qualifier == 'l') {
qualifier = 'L';
fmt.str++;
} else if (qualifier == 'h') {
qualifier = 'H';
fmt.str++;
}
}
}
/* default base */
spec->base = 10; spec->base = 10;
switch (*fmt.str) { fmt.state = FORMAT_STATE_SIZE(int);
case 'c': static const struct format_state {
fmt.state = FORMAT_STATE_CHAR; unsigned char state;
fmt.str++; unsigned char flags_or_double_state;
return fmt; unsigned char modifier;
unsigned char base;
} lookup_state[256] = {
// Qualifiers
['l'] = { FORMAT_STATE_SIZE(long), FORMAT_STATE_SIZE(long long), 1 },
['L'] = { FORMAT_STATE_SIZE(long long), 0, 1 },
['h'] = { FORMAT_STATE_SIZE(short), FORMAT_STATE_SIZE(char), 1 },
['H'] = { FORMAT_STATE_SIZE(char), 0, 1 }, // Questionable, historic
['z'] = { FORMAT_STATE_SIZE(size_t), 0, 1 },
['t'] = { FORMAT_STATE_SIZE(ptrdiff_t), 0, 1 },
case 's': // Non-numeric formats
fmt.state = FORMAT_STATE_STR; ['c'] = { FORMAT_STATE_CHAR },
fmt.str++; ['s'] = { FORMAT_STATE_STR },
return fmt; ['p'] = { FORMAT_STATE_PTR },
['%'] = { FORMAT_STATE_PERCENT_CHAR },
case 'p': // Numerics
fmt.state = FORMAT_STATE_PTR; ['o'] = { 0, 0, 0, 8 },
fmt.str++; ['x'] = { 0, SMALL, 0, 16 },
return fmt; ['X'] = { 0, 0, 0, 16 },
['d'] = { 0, SIGN, 0, 10 },
['i'] = { 0, SIGN, 0, 10 },
['u'] = { 0, 0, 0, 10, },
case '%':
fmt.state = FORMAT_STATE_PERCENT_CHAR;
fmt.str++;
return fmt;
/* integer number formats - set up the flags and "break" */
case 'o':
spec->base = 8;
break;
case 'x':
spec->flags |= SMALL;
fallthrough;
case 'X':
spec->base = 16;
break;
case 'd':
case 'i':
spec->flags |= SIGN;
break;
case 'u':
break;
case 'n':
/* /*
* Since %n poses a greater security risk than * Since %n poses a greater security risk than
* utility, treat it as any other invalid or * utility, treat it as any other invalid or
* unsupported format specifier. * unsupported format specifier.
*/ */
fallthrough; };
default: const struct format_state *p = lookup_state + (u8)*fmt.str;
WARN_ONCE(1, "Please remove unsupported %%%c in format string\n", *fmt.str); if (p->modifier) {
fmt.state = FORMAT_STATE_INVALID; fmt.state = p->state;
if (p->flags_or_double_state && fmt.str[0] == fmt.str[1]) {
fmt.state = p->flags_or_double_state;
fmt.str++;
}
fmt.str++;
p = lookup_state + *fmt.str;
if (unlikely(p->modifier))
goto invalid;
}
if (p->base) {
spec->base = p->base;
spec->flags |= p->flags_or_double_state;
fmt.str++;
return fmt;
}
if (p->state) {
fmt.state = p->state;
fmt.str++;
return fmt; return fmt;
} }
if (qualifier == 'L') invalid:
fmt.state = FORMAT_STATE_SIZE(long long); WARN_ONCE(1, "Please remove unsupported %%%c in format string\n", *fmt.str);
else if (qualifier == 'l') { fmt.state = FORMAT_STATE_INVALID;
fmt.state = FORMAT_STATE_SIZE(long);
} else if (qualifier == 'z') {
fmt.state = FORMAT_STATE_SIZE(size_t);
} else if (qualifier == 't') {
fmt.state = FORMAT_STATE_SIZE(ptrdiff_t);
} else if (qualifier == 'H') {
fmt.state = FORMAT_STATE_SIZE(char);
} else if (qualifier == 'h') {
fmt.state = FORMAT_STATE_SIZE(short);
} else {
fmt.state = FORMAT_STATE_SIZE(int);
}
fmt.str++;
return fmt; return fmt;
} }